Glossary | Computer Networking First-Step

1s digit: In mathematics, in a number with multiple digits or numerals, this is the digit on the far-right end of the multidigit number. This digit represents the value of the digit times 1.

4-wire circuit: A reference to the fact that a leased line typically uses two pairs of wires.

10 GigE: An abbreviation of the term 10 Gigabit Ethernet.

10 Gigabit Ethernet: An Ethernet standard that transmits data at 10 billion bits per second, or 10 Gigabits per second.

10/100 NIC: An Ethernet NIC that can run at either 10 Mbps or 100 Mbps, including autonegotiation of the speed and duplex setting.

10BaseT: A name for 10 Mbps Ethernet when using twisted-pair cabling.

10s digit: In mathematics, in a decimal number with multiple digits or numerals, this is the digit second from the right end of the multidigit number. This digit represents the value of the digit times 10.

100s digit: In mathematics, in a decimal number with multiple digits or numerals, this is the digit third from the right end of the multidigit number. This digit represents the value of the digit times 100.

100BaseT: A name for 100 Mbps Ethernet when using twisted-pair cabling.

802.2: The IEEE committee that defined common features for several types of LANs, including Ethernet. Also known as Logical Link Control (LLC).

802.3: The IEEE committee that defined Ethernet-specific details of Ethernet. Also known as Media Access Control (MAC).

AAA: The process of authentication, authorization, and accounting. Pronounced "triple A."

AAA server: An authentication, authorization, and accounting server. This term is typically used when the AAA functions are being performed as users access an ISP.

access link: The leased line between the customer site and a local CO that connects to a router to a Frame Relay service.

access rate: The speed at which the access link is clocked. This choice affects the price of the Frame Relay connection.

accounting: In the context of network security, the process of recording and reporting events that occur as part of the authorization and authentication processes.

acknowledgment number: A field in the TCP header that identifies the number of the next byte of data that the computer sending the TCP segment expects to receive next. It is used as part of the error recovery process.

Address Resolution Protocol: See [ARP]

addressing: The process of putting a number in a header, with that number defining where to send the data in the packet.

amplitude: The distance in a graph between the X-axis and the highest point on the graphed curve.

analog electrical signal: An electrical signal whose voltage level changes continuously. When graphed over time, the curve gently changes between the maximum and minimum voltage levels, creating a continuous curve, which works well for transmitting voice traffic.

analog modem: See [modem]

antivirus software: Software that typically resides on the end user computer; it examines all files that are copied to the computer, typically by e-mail or web browsers, and removes files that contain computer viruses.

ARP (Address Resolution Protocol): The protocol by which any IP host can, given an IP address on the same LAN subnet, discover the LAN address used by that other IP host.

ARP broadcast: A LAN broadcast frame that holds an ARP request. The request has an IP address listed; if the host who uses that IP address gets the request, it should reply, stating its LAN MAC address in an ARP reply message.

ARP cache: A table on each IP host, including routers, which holds the IP address and MAC address mappings learned using the ARP protocol. (Note that "cache" is pronounced just like the word "cash.")

ARP reply: A LAN unicast frame that holds an ARP reply. The reply has an IP address listed and the MAC address used on the same LAN interface. It is sent in reply to an ARP broadcast.

asymmetric transmission rates: The use of different transmission speeds depending on the direction that traffic is sent.

authentication: The process of verifying the identity of a user, typically by exchanging usernames and passwords.

authentication server: A server that holds a list of usernames and passwords for the purpose of allowing other servers to query the authentication server as to whether a particular username/password combination is valid.

authoritative DNS server: The DNS server that knows the names and IP addresses that are related to a particular domain.

authorization: The process of deciding what resources a particular user can access.

autonegotiation: A process by which a NIC and a switch port can choose the Ethernet speed to use, as well as whether to use full duplex.

bandwidth: Jargon that generally refers to the speed at which bits can be sent across a physical link.

BGP (Border Gateway Protocol): An IP routing protocol, defined in RFCs, making it a public protocol. BGP is used mainly to exchange routing information between different companies, including between ISPs and their customers.

binary digit: In mathematics, a digit or numeral that is part of a number based on the binary numbering system.

binary numbering system: In mathematics, a convention for numbering things in which 2 separate numerals are used, namely 0 and 1.

bit (binary digit): A single binary number, either 0 or 1, as stored by a computer in memory.

bits per second (bps): See [bps]

Border Gateway Protocol (BGP): See [BGP]

bps (bits per second): Units used to describe the speed at which data is sent over some communication media, as measured by the number of bits sent during a single second.

broadcast address: An Ethernet MAC address that represents all devices on a LAN. Switches flood frames that are sent to the broadcast address.

broadcast domain: A set of devices for which a broadcast frame sent by one device will be received by all devices in the same broadcast domain.

buffer: (Noun) Storage location inside a switch in which frames can be stored when the switch is waiting for the output port to become available. (Verb) The switch process of storing frames in memory.

bunch of bits: Jargon used in this book to describe some information, in binary form, that sits in a computer.

byte: 8 consecutive bits as stored on a computer.

cable: A long, somewhat cylindrical piece of plastic, often used to enclose some wires.

canonical: A shortened version of the term canonical format.

canonical format: A term referring to the format of TCP/IP addresses when written in decimal.

carrier sense multiple access collision detect (CSMA/CD): See [CSMA/CD]

Challenge Handshake Authentication Protocol (CHAP): See [CHAP]

channel service unit/data service unit (CSU/DSU): See [CSU/DSU]

CHAP (Challenge Handshake Authentication Protocol): A component protocol of PPP that is used to pass username and password information, sending the password as a message digest instead of clear text.

CIR (committed information rate): The rate at which the router can send data for an individual Frame Relay PVC, for which the provider commits to deliver that amount of data. The provider sends any data in excess of this rate for this PVC if its network has capacity at the time. This choice typically affects the price of each PVC.

class of network: In IP addressing, this term refers to whether a particular IP network is a Class A, Class B, or Class C type of network.

clear text: Information that is in its original form, with no attempt to encode, encrypt, or change the information before transmission.

client: A generic term used to refer to software on an end user's computer, or to the computer itself. The term refers to the fact that the end user typically wants to use one or more network services, so the end user is a client or customer of those services.

clock rate: A term that refers to the speed at which bits are sent over a physical link. Often used to describe the speed setting on a CSU/DSU.

clocking: The act of making a WAN link operate at the speed agreed upon with the telephone company.

CO (central office): An office or building in which the telephone company installs its equipment.

collision: In Ethernet, a condition in which two (or more) frames are transmitted over the same pair of wires simultaneously.

co-lo (co-location): A practice in which a telco cooperates with an ISP, allowing the ISP to locate its equipment inside the telco's CO.

co-location (co-lo): See [co-lo]

commercial driver's license: A license to drive on public roads that also allows the driver to drive certain types of commercial vehicles.

committed information rate (CIR): See [CIR]

computer virus: See [virus]

configuring a router: A process by which someone types in commands that tell a router what to do. For instance, an engineer would configure a router to tell the router which IP addresses to use on each interface.

content (web content): A term referring to the text, graphics, images, video, and audio that comprise a web page.

cracker: A person who attempts to cause problems with networks and computers.

cross-over cable: An Ethernet LAN cable for which the wire in pin 1 of the connector on one end of the cable connects to pin 3 on the other end of the cable. The wire in pin 2 on one connector is connected to pin 6, pin 3 connects to pin 1, and pin 6 connects to pin 2. By doing so, the twisted pair on pins 1 and 2 connect to pins 3 and 6 on the other end of the cable, and the twisted pair at pins 3 and 6 connect to pair 1 and 2 at the other end.

CSMA/CD (carrier sense multiple access collision detect): Algorithm that defines how Ethernet-attached computers should behave in order to reduce collisions and to recover from collisions when they do occur.

CSU/DSU (channel service unit/data service unit): A device or a component of a router serial interface that performs many functions, including clocking a serial link at the appropriate transmission speed.

data-link connection identifier (DLCI): See [DLCI]

decimal digit: In mathematics, a digit or numeral that is part of a number based on the decimal numbering systemnamely 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.

decimal numbering system: In mathematics, a convention for numbering things in which ten separate numerals are used.

DECnet: DECnet was Digital Equipment Corporation's proprietary networking model.

decrypting: The process of re-creating the original data that was transmitted over a network after being encrypted by the sender of the data.

default gateway: See [default router]

default router: A setting on a TCP/IP host that refers to the IP address of a router. The host sends packets to this default router, expecting the router to then be able to forward the packet to the destination.

default web page: See [home page]

demodulation: The act of interpreting a received analog electrical signal to decode a binary 0 or 1.

denial of service attack: An action that a cracker takes to cause a service or server to have problems, or possibly fail, which then denies legitimate users the opportunity to use the service.

Destination Address field: The 6-byte long portion of an Ethernet header in which a NIC places the intended recipient's Ethernet address before sending a frame.

destination IP address: A field in the IP header that lists the IP address of the intended recipient of the packet.

digit: In mathematics, a single symbol that can be used to represent part of a number; for instance, the digit "1" is part of the number "321." Another word for numeral.

digital subscriber line (DSL): See [DSL]

directly connected route: A route that refers to a network or subnet that is connected to an interface on that router.

directly connected subnet: A subnet for which a router has a working physical interface connected to the physical network where the subnet resides.

directory assistance: A service from a phone company by which you call a well-known phone number and get help in finding someone's phone number.

disk drive: A component of a computer that can permanently hold the contents of computer files.

DIX Ethernet: A term referring to the original Ethernet standard, as developed by people at Digital Equipment Corporation, Intel, and Xerox.

DLCI (data-link connection identifier): A Frame Relay address used in Frame Relay headers to identify the PVC.

DMZ: A LAN where servers are installed in an enterprise network, with those servers available to users on the Internet. This allows separation of servers intended for internal use only (on the inside) from the servers that are available to the general public (on the DMZ).

DMZ interface: An interface on a firewall that is connected to the DMZ LAN. It is considered to be more secure than the outside but less secure than the inside.

DNS (Domain Name System): The protocols and conventions that define a worldwide, distributed TCP/IP host name database, which allows a client computer to use host names, request name resolution, and get a correct response that identifies the IP address that corresponds to the name.

DNS resolution request: A DNS protocol message, sent by an end user host to a DNS server, that supplies a TCP/IP host name and requests the corresponding IP address.

DNS resolver: A TCP/IP host that makes a DNS resolution request, asking for resolution between a host name and the corresponding IP address.

DNS server: A computer running DNS server software, providing name resolution services.

domain: The set of computers whose names all end with the same domain name.

domain name: A term referring to a suffix of a host name that identifies the authority, organization, or company that controls all names that end in that suffix.

Domain Name System (DNS): See [DNS]

Dot 1 Q: Jargon referring to the IEEE 802.1Q standard of VLAN trunking.

dot-com: Jargon that refers to businesses that are primarily Internet based. The term refers to the top-level domain (.com) of their respective domain names.

dotted decimal format: A term referring to the format of TCP/IP addresses when written in decimal.

download: A file transfer in which the computer receives or gets a file that it did not previously have a copy of.

DSL (digital subscriber line): Public network technology that delivers high bandwidth over conventional copper wiring at limited distances. The most common types of DSL are ADSL, HDSL, SDSL, and VDSL. Because most DSL technologies do not use the complete bandwidth of the twisted pair, there is room remaining for a voice channel.

DSL access multiplexer (DSLAM): See [DSLAM]

DSL modem: A device that generates and receives data over a local phone line.

DSLAM (DSL access multiplexer): A device, typically found at a telco CO, that splits the digital signals sent by DSL modems to a router and splits the voice signals out to a telephone switch.

dynamically assigned port: A port number that is chosen at the TCP client computer. The computer simply chooses a currently unused port number.

EIGRP (Enhanced Interior Gateway Routing Protocol): An IP routing protocol, defined by Cisco, making it proprietary. It is an enhanced version of IGRP.

electrical cabling: Cabling that includes copper wires inside the cable so that electricity can be sent over the wires.

electromagnetic interference (EMI): See [EMI]

electronic mail (e-mail): See [e-mail]

e-mail (electronic mail): The process of creating, sending, and receiving messages electronically.

e-mail address: A text string that identifies an individual e-mail user. The address has two partsa username and a computer nameseparated by an @ sign, such as user1@isp1.net.

e-mail client: Software, running on an end user computer, that provides a user interface to the end user to create and read e-mail. It also implements the protocols required to transmit and receive the e-mail.

e-mail server: Software, running on some server computer hardware, that receives and forwards e-mail in a network. It's much like what the postal service does for paper snail mail.

EMI (electromagnetic interference): The effect by which an electrical current running over a wire induces a magnetic field, outside the wire, which in turn can induce an electrical current in another nearby wire. In layman's terms, this phenomenon interferes with the electricity in nearby wires, and when transmitting data, this effect might cause errors in the data transmitted over that nearby wire.

encapsulation: The process of taking data from a higher-layer protocol and adding a header and possibly a trailer.

encode: The process of varying the characteristics of an electrical signal to transmit binary values across a cable.

encoding standard: A particular standard that defines what electrical characteristics mean binary 0 or binary 1.

encryption: The process of performing a mathematical formula against some data for the purpose of hiding the original data.

encryption key: A secret set of characters used to encrypt and decrypt data.

Enhanced Interior Gateway Routing Protocol (EIGRP): See [EIGRP]

enterprise network: A network created for use by a single company or organization.

enterprise WAN: A WAN used by a single company or enterprise to connect LANs at different sites.

ephemeral port: See [dynamically assigned port]

error detection: The process of realizing that a received frame had some bits changed in it as a result of traveling across the network.

error recovery: The process of causing the retransmission of lost or in-error data.

Ethernet: A widely popular standard for LAN communications.

Ethernet address: A 6-byte long hexadecimal number used to identify a NIC that is attached to an Ethernet.

Ethernet hub: A networking device that allows multiple devices to be cabled to it. The hub repeats incoming electrical signals out all other physical ports, thereby requiring only a single cable to each device, while allowing all devices to communicate with all other devices.

Ethernet version 2: Another name for DIX Ethernet.

Exterior Routing Protocol: A routing protocol that is designed for use between different companies and organizations.

external CSU/DSU: A CSU/DSU that sits outside a router, with a cable between the router serial interface and the CSU/DSU.

Fast Ethernet (FE): See [FE]

FCS (frame check sequence): A field in an data link protocol trailer that allows the receiver of a frame to determine whether the frame had errors during transmission.

FE (Fast Ethernet): Defined by IEEE 802.3u, FE specifies Ethernet with a transmission speed of 100 Mbps.

field: A particular part of a header or trailer in which a specific piece of information is carried. For instance, the Ethernet destination address field in an Ethernet header is a field inside the header.

file read: A process that a computer uses to look for a file, find it, and give the data inside the file to an application program.

file server: A computer that performs file services.

file services: The process by which one computer, typically called a file server, keeps files on its disk drive and allows other computers to read and write the files by using the network.

file transfer: A network application that allows a user to copy files from one computer to another. Whereas download usually means copying into a computer, file transfer typically means the file can be copied in either direction between two computers.

File Transfer Protocol (FTP): See [FTP]

file write: A process that a computer uses to replace an existing file's content.

firewall: A networking device that sits in the path through which packets are forwarded, examining the packets, and allowing only the packets that are determined to be acceptable based on the security policy of a network.

flooding: The process whereby a switch forwards a frame out all ports except the port in which the frame arrived at the switch.

flow: A series of IP packets that go from one IP host to another, and vice versa. Sometimes used as a generic term to refer to a TCP connection.

forward acknowledgment: The process of acknowledging data by setting the acknowledgement field to the next byte of data that should be received, as opposed to setting the acknowledgement field to the last byte of data that was received.

forward versus filter: A term referring to a switch's decision to forward a frame on one port, and not to forwardin other words, to filterthe frame on other ports.

forwarding: When this term is used in the context of IP routing, it is another term for routing.; See also [routing]

forwarding decision: When this term is used in the context of IP routing, it refers to the process of matching a router's routing table and making a decision of where to forward the packet.

frame: A bunch of bits that includes some data in the middle, with a header in front of the data, and a trailer after the data. This term specifically refers to headers and trailers as defined by OSI data link layer protocols.

frame check sequence (FCS): See [FCS]

Frame Relay: A data link layer protocol defined in part by the Frame Relay Forum, ANSI, and the ITU. These protocols define how routers and service providers together create Frame Relay networks.

Frame Relay Forum: A vendor consortium that defined the earliest Frame Relay protocols, before ANSI and ITU created Frame Relay standards.

Frame Relay network: The cabling and equipment that together create the capability to forward Frame Relay frames between two devices.

Frame Relay service: A telco-provided service that creates PVCs between pairs of customer sites.

Frame Relay service provider: A business, typically a telco, that provides Frame Relay services.

Frame Relay switch: A generic term referring to a type of equipment, found in the COs of Frame Relay service providers, that can be configured to forward Frame Relay frames.

frequency: The number of times per second for which a repetitive electrical signal repeats itself.

FTP (File Transfer Protocol): A TCP/IP protocol specification used to allow file sharing by copying files to and from a server.

FTP client: Software, running on an end user computer, that provides a user interface to the end user to get files to and from an FTP server. It also implements the FTP application layer protocols required to transmit and receive the e-mail.

FTP Control Connection: A connection between an FTP client and server over which the client sends all control information, such as changing the directory, naming the file, and requesting a GET or a PUT operation.

FTP data connection: A connection between an FTP client and server over which the actual data is transferred.

FTP GET: See [GET]

FTP PUT: See [PUT]

FTP server: Software, running on some server computer hardware, that allows FTP clients to connect to it for the purpose of getting files from the server and putting files on the server so that others can come and get them.

full duplex: The process of both sending and receiving data on a physical interface at the same point in time.

full mesh: In Frame Relay, a network in which all pairs of Frame Relay-connected devices have a PVC between them.

gateway: A term for a myriad of networking devices that generally convert from one standard to another.

Gbps: Acronym for gigabit per second.

GE (Gigabit Ethernet): Defined by IEEE 802.3z and IEEE 802.3ab, it specifies Ethernet with a transmission speed of 1000 Mbps, or 1 Gbps.

GET: FTP term meaning that the FTP client is moving a file from the server to itselfin other words, it's getting the file from the server.

Gigabit Ethernet (GigE): See [GigE/GE]

gigabit per second (Gbps): See [Gbps]

GigE (Gigabit Ethernet): Defined by IEEE 802.3z and IEEE 802.3ab, it specifies Ethernet with a transmission speed of 1000 Mbps, or 1 Gbps.

hacker: A person who attempts to access networks and computer systems, but without malicious intent.

half duplex: Logic used by a device on a LAN, for which it chooses to either send or receive a frame at a point in time, but not both at the same time. CSMA/CD logic imposes half duplex logic on a NIC.

HDLC (high-level data link control): A data link layer protocol that is used on WAN links.

header: A bunch of bits placed in front of data prior to transmission. The bits allow some protocol to have a place to include information along with the user data to accomplish the function of the protocol.

high-level data link control (HDLC): See [HDLC]

home page: When a web browser requests a web page from a web server, if the URL does not include text after the name of the server, the server returns the default web page. That default web page is called a home page.

host name: A name that represents a TCP/IP host computer.

host part: The portion of an IP address that has a unique value as compared to all other hosts inside the network. Class A, B, and C networks have 3, 2, and 1 octet long host parts, respectively.

host-based IDS: IDS software that is installed on a computer, typically a server.

HTML (Hypertext Markup Language): A standard, which is not part of the TCP/IP model, that defines the format and meaning of one type of object used to build a web page.

HTTP (Hypertext Transfer Protocol): A TCP/IP protocol, defined in RFC 2616, that defines the protocols through which a web browser identifies the web pages and web objects it wants to download, as well as the processes by which the browser downloads the pages and objects.

Hypertext Markup Language (HTML): See [HTML]

Hypertext Transfer Protocol (HTTP): See [HTTP]

IDS(intrusion detection system): A device or software that examines packets that are allowed by a firewall, looking for things that a cracker might use to deny service or to place a virus on a computer.

IDS signature: A set of characterizations of the types of packets that crackers have used in the past to cause viruses or to deny service.

IEEE (Institute of Electrical and Electronics Engineers): A standards organization that defines standards for a large variety of topics, including computer networking (http://www.ieee.org).

IEEE 802.1Q: The IEEE-defined standard VLAN trunking protocol.

IEEE 802.3ab: A name for an IEEE committee, as well as a set of standards, relating to Gigabit Ethernet. Defines Gigabit Ethernet over copper cabling.

IEEE 802.3ae: A name for an IEEE committee, as well as a set of standards, relating to 10 Gigabit Ethernet. Defines 10 Gigabit Ethernet over optical cabling.

IEEE 802.3z: A name for an IEEE committee, as well as a set of standards, relating to Gigabit Ethernet. Defines Gigabit Ethernet over optical cabling.

IEEE Ethernet: Jargon referring to Ethernet standards as defined by the IEEE.

IETF (Internet Engineering Task Force): The standards body that creates and manages TCP/IP standards. The IETF, to quote their website (http://www.ietf.org), is "a large, open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual."

IGRP (Interior Gateway Routing Protocol): An IP routing protocol, defined by Cisco, making it proprietary.

information: In the context of services provided by a telephone company, this is the same thing as directory assistance.; See also [directory assistance]

inside interface: An interface on a firewall connected to the LAN that is considered to be the most secure of all the LANs connected to the firewall.

Institute of Electrical and Electronics Engineers (IEEE): See [IEEE]

interconnected networks: See [internetwork]

Interior Gateway Routing Protocol (IGRP): See [IGRP]

Interior Routing Protocol: A routing protocol designed for use inside a single company or organization.

internal CSU/DSU: A CSU/DSU integrated onto a router serial interface card, which is then installed in a router.

International Organization for Standardization (ISO): See [ISO]

International Telecommunication Union (ITU): See [ITU]

Internet: The global TCP/IP network to which almost every company and organization in the world is connected. The Internet allows communication between a multitude of computers on the planet.

Internet draft: A document that describes a protocol or standard that is under development but not yet approved.

Internet Engineering Task Force (IETF): See [IETF]

Internet Explorer: The name of a popular web browser software product written by Microsoft Corporation.

Internet Message Format RFC: A TCP/IP protocol, defined in RFC 2282, that defines the headers placed around e-mail messages when they are being sent through a network.

Internet Protocol (IP): See [IP]

Internet service provider (ISP): See [ISP]

internetwork (interconnected networks): Multiple networks that are connected using routers.

Inter-Switch Link (ISL): See [ISL]

intrusion detection system (IDS): See [IDS]

Inverse ARP: A process and protocol that allows a router to announce its IP address with a message sent over a Frame Relay PVC.

IP (Internet Protocol): A TCP/IP protocol, defined in RFC 791, that defines logical IP addressing and routing.

IP address: A 32-bit number, usually written in dotted decimal format. It identifies a network interface on a computer to the IP protocol, and it transmits and receives IP packets.

IP header: Information that the IP protocol adds to the front of a transport layer segment to create an IP packet. The header is typically 20 bytes long and includes the source and destination IP addresses.

IP host: Any computer that has at least one IP address.

IP network: A grouping of IP addresses for which some initial portion of their IP address values are in common.

IP network number: A dotted-decimal number that represents a single network. The number has the same value as the IP addresses inside the network part of the number, and all decimal 0s in the host part of the number.

IP packet: An IP header, along with any encapsulated data after the IP header.

IP routing table: See [routing table]

ISL (Inter-Switch Link): The Cisco-proprietary VLAN trunking protocol.

ISO (International Organization for Standardization): Organization that developed the OSI model and that continues to work with standards today. To quote the website (http://www.iso.ch), the ISO is "a network of national standards institutes from 147 countries working in partnership with international organizations, governments, industry, business, and consumer representatives. A bridge between public and private sectors."

ISP (Internet service provider): A company that provides a service of network connection to the Internet. ISPs support individual users as well as corporations.

ITU (International Telecommunication Union): A standards organization composed of telecommunications companies from around the world. The ITU defines standards for a large variety of topics, including telephony and wide-area networks (http://www.itu.org).

jamming signal: An electrical signal specified by Ethernet standards that is sent when a collision occurs to ensure that all devices realize a collision took place.

Kbps (kilobits per second): Units that describe the speed at which data is sent over some communication media. Kbps represents 1000 bits in 1 second; for example, 2 kbps is 2000 bits per second.

Kerberos: A TCP/IP standard protocol used to exchange AAA information between application servers and authentication servers.

key: See [encryption key]

kilobits per second (kbps): See [Kbps]

LAN (local-area network): A network that by definition includes components that are relatively near to each other, typically in the same building.

LAN legal: A term created just for this book to make an analogy to having a car be "street legal."

LAN switch: A networking device that forwards frames based on their destination address.

layer: A subset of protocols of a single networking model. The protocols in the same layer have the same general or related goals.

Layer 8: A sarcastic term referring to business, political, or other nontechnical issues that might affect network design choices. It refers to a nonexistent OSI layer, just above the highest OSI layer.

learning: With LAN switches, "learning" refers to the process of examining the source MAC address of frames that the switch received, and adding new entries to the MAC address table. The switch adds an entry listing the source MAC address of the frame, along with the port in which the frame was received.

leased circuit: See [WAN link]

leased line: A service from telephone companies that provides a communication path between a pair of sites.

lightning bolt: A style of drawing a line so that it looks like a bolt of lightning. WAN links are often drawn with a lightning bolt line style in network diagrams.

LLC (Logical Link Control): Another name for 802.2.

local-area network (LAN): See [LAN]

local host file: A file on an individual computer that holds a list of host names and their corresponding IP addresses. A host computer can look at this local host file to perform name resolution.

local loop: A cable from the premises of a telephone subscriber to the telephone company CO.

Logical Link Control (LLC): See [LLC]

loopback circuit: On an Ethernet NIC, hardware that causes the electrical signal sent by the NIC to also be received on the NIC, without passing over the cable.

MAC (Media Access Control): Another name for 802.3.

MAC address: Another name for Ethernet Address.

MAC address table: A table containing a list of MAC addresses and the physical switch ports out which a switch should forward frames that are destined to each MAC address. LAN switches use a MAC address table to make good forwarding and filtering decisions.

maximum segment size (mss): See [mss]

Mbps (megabits per second): Units that describe the speed at which data is sent over some communication media. Mbps represents 1,000,000 bits in 1 second; for example, 2 mbps is 2000 kbps, or 2,0000,000 bits per second.

Media Access Control (MAC): See [MAC]

megabits per second (Mbps): See [Mbps]

message digest: The result of a mathematical formula that is used to hide a password during transmission.

metric: A number that represents how good a particular route to a particular destination is. Lower metric routes are better than routes with larger metrics.

microphone: A device that converts sounds into analog electrical signals.

modem (modulator-demodulator): A device that converts digital and analog signals for the purpose of communicating over analog telephone lines.

modulation: The act of changing an analog electrical signal to encode a binary 0 or 1.

modulator-demodulator (modem): See [modem]

mss (maximum segment size): The largest TCP segment that a computer is allowed to send.

name resolution: The process of finding an IP address that corresponds to a known name.

Netscape: The name of a popular web browser software product, more fully named Netscape Navigator. The product was originally written by a company called Netscape and is now owned by AOL, Inc.

network analysis tool: A device that connects to a network, collects copies of frames passing over the network, and allows the user to analyze and read the contents of the frames.

network-based IDS: An IDS that sits in the network, monitoring the packets that pass over a LAN.

network cloud: A style of drawing diagrams, with a cloud representing all or part of a network.

network interface: The lowest layer of the TCP/IP architectural model, matching both Layer 1 and Layer 2 of the OSI model.

network interface card (NIC): See [NIC]

network part: The portion of an IP address that has the same value as all the other addresses in the same network. Class A, B, and C networks have 1, 2, and 3 octet long network parts, respectively.

network utility: The idea of treating the basic ability to communicate between two computers just like you think of the telephone, electrical power, water, and so on.

networking protocol: A well-defined set of rules regarding the behavior of a networking device, networking cable, or networking software. Often synonymous with networking standard, this term typically refers to a process.

networking rulebook: This term is actually just used in this book to make a point. In concept, the rulebook lists a set of standards and protocols so that if all the devices conform to these standards, the network will work.

networking standard: A well-defined set of rules regarding the characteristics of a networking device, networking cable, or networking software.

next-hop router: A field in the IP routing table that identifies the IP address of the next router that needs to receive a packet to ensure delivery to the correct destination.

NIC (network interface card): A computer card that gives a computer the ability to send and receive data across a physical network. The term comes from the idea that the card is the PC's interface to the network.

numeral: In mathematics, a single symbol that can represent part of a number; for instance, the digit "1" is part of the number "321." Another word for digit.

object: In the context of WWW, an object is a single file that can be transferred using HTTP. Web pages consist of one or more objects, with more complex single web pages containing hundreds of objects.

octet: A different generic term that can be used instead of the word byte. Often used to describe each byte of IP addresses.

Open Shortest Path First (OSPF): See [OSPF]

Open Systems Interconnection (OSI): See [OSI]

operating system (OS): See [OS]

optical cabling: Cabling that uses glass fibers instead of copper wires, with light being sent over the glass fiber, rather than electricity, to encode bits for transmission.

OS (operating system): Software whose purpose is to control the general operation of a computer. For PCs, Microsoft Windows variants are the most popular operating systems.

OSI (Open Systems Interconnection): A public networking model. Few computers use OSI today. The International Organization for Standardization (ISO) manages OSI protocols.

OSPF (Open Shortest Path First): An IP routing protocol, defined in RFCs, making it a public protocol.

outgoing interface: A field in the IP routing table that identifies this router's interface out which a packet should be sent for it to be delivered to the correct destination.

outside interface: An interface on a firewall that is connected to a LAN. It is considered to be the least secure of all the LANs that are connected to the firewall.

packet: A group of bits that are combined for transmission in a network. This term specifically refers to the header and data defined by OSI layer 3 (network) layer protocols.

PAP (Password Authentication Protocol): A component protocol of PPP, used to send usernames and passwords as clear text.

partial mesh: In Frame Relay, a network in which not all pairs of Frame Relay-connected devices have a PVC between them.

password: A string of characters, kept private from other people, that is used to prove that a user is who he claims to be.

Password Authentication Protocol (PAP): See [PAP]

patch cable: A short LAN cable used as one of several cables in a structured wiring system. For example, the cable from a PC to a wall plate is a patch cable.

patch panel: A convenient place for an electrician to connect the wires in cables that terminate in a wiring closet. The wires are connected to one side of the panel. The panel provides an electrical pathway to other plugs, or receptacles, much like the ones in the wall plate in a cubicle. The patch panel allows an electrician, or network engineer, to use a short patch cable to connect those pairs of wires to other devices.

PC (personal computer): A computer that a singer user typically uses. It usually has one keyboard, one display, and one mouse.

permanent virtual circuit (PVC): See [PVC]

personal computer (PC): See [PC]

phone switch: See [telco switch]

physical connectivity: Generic term referring to the combination of cabling, networking devices, and network interface cards (NICs) in the computers, which together provide the physical capability to transmit and receive data across a network.

physical LAN: A broadcast domain created by a combination of computers, cables, and networking devices.

physical medium: A cable with either copper wires or optical fibers over which electrical (copper) or light (fiber) energy can be passed. Binary data can be transmitted over such media through encoding conventions that define what particular electrical or light energy implies a binary 0 or 1.

pin: The physical position at the end of a connector where the copper part of the wire sits.

PIX Firewall: The name Cisco uses to brand its firewall product line.

point of presence (POP): See [POP]

point-to-point link: See [WAN link]

Point-to-Point Protocol (PPP): See [PPP]

POP (point of presence): A building that houses equipment that an ISP owns.

POP3 (Post Office Protocol, Version 3): A TCP/IP protocol, defined in RFC 1939, that defines the messages that control the transfer of e-mail. POP3 is typically used only by clients, and only when retrieving e-mail from a POP3 server.

POP3 Server: Another term for POP3 Server Software.

POP3 Server Software: Software that implements the POP3 server features.

Post Office Protocol, Version 3 (POP3): See [POP3]

postal code: Generic term for what is called a zip code in the U.S.; See also [zip code]

PPP (Point-to-Point Protocol): A data link layer protocol that is used on WAN links.

preamble: In Ethernet, a series of alternating binary 1s and 0s at the beginning of a new frame.

print server: A computer that performs print services.

print services: The process by which one computer, typically called a print server, allows other computers to send it files that are then printed on a printer that is physically attached to the print server.

profiling: In law enforcement, a characterization of how criminals typically operate, so that law enforcement personnel can more easily catch the criminals.

PSTN (Public Switched Telephone Network): A general term referring to the variety of telephone networks and services in place worldwide. Sometimes called plain old telephone service (POTS).

public networking model: A networking model for which no one vendor dictates the standards and protocols, with individuals from many companies and organizations participating in the standards definition process.

Public Switched Telephone Network (PSTN): See [PSTN]

PUT: FTP term meaning that the FTP client is moving a file from itself to the serverin other words, it's putting the file on the server.

PVC (permanent virtual circuit): A predefined Frame Relay VC. A PVC can be equated to a leased line in concept.

RADIUS: A TCP/IP standard protocol that routers and AAA servers use to exchange AAA information.

remainder: In an algorithm in Appendix B, "Converting IP Addresses Between Decimal and Binary," this term represents the result of subtracting one number from another.

Request for Comments (RFC): See [RFC]

resolver: A term that refers to a TCP/IP host that is asking for help in resolving a host name to its IP address.

RFC (Request for Comments): Each RFC defines some protocol or standard that is important to the TCP/IP model. An RFC has been through several reviews by the IETF. The term RFC comes from the fact that anyone can comment on the protocol while it is being reviewed. In fact, the document is posted on the Internet so that anyone can look at it and comment before it becomes an RFC.

right-of-way: Legal term that refers to the right to disrupt normal access to some physical location to do some work. For example, the telephone company can dig up the street to put in new cables because it has the right-of-way.

RIP (Routing Information Protocol): An IP routing protocol, defined in RFCs.

root DNS server: A DNS server that lists domain names and their corresponding DNS server IP addresses. They are used to refer DNS requests to the authoritative DNS server for a domain.

router: A networking device with multiple network interfaces, whose purpose is to forward IP packets from one physical network to another. Routers provide an important packet-forwarding function for the TCP/IP internetworking layer to perform end-to-end delivery of IP packets.

routing: The process of receiving IP packets, making a decision of where to send the packet next, and forwarding the packet.

Routing Information Protocol (RIP): See [RIP]

routing protocol: A type of networking protocol designed to allow routers to exchange routing information with each other, with the end goal of having all the routers eventually learn routes to all IP subnets and networks in an internetwork.

routing table: A table that routers use so that they know where to forward packets.

routing update: A message defined by a routing protocol that contains the routing information that a router wants to share with other routers.

sales engineer (SE): See [SE]

SE (sales engineer): A person who sells a company's products or services, with focus on the engineering or technical aspects of the sale.

segment: See [TCP segment]

segmentation: The process by which an application gives TCP some data, with TCP breaking the data into smaller pieces, called segments, for transmission into a network.

sequence number: A value in the TCP header that is used to number the packets, for the purpose of noticing lost or in-error packets so that bad packets can be resent.

serial interface: A type of physical interface that a router uses to connect to a WAN link.

serial link: See [WAN link]

shielded twisted-pair (STP): See [STP]

signature: See [IDS signature]

Simple Mail Transfer Protocol (SMTP): See [SMTP]

SMTP (Simple Mail Transfer Protocol): A TCP/IP protocol, defined in RFC 2821, that defines the messages used to control the transfer of e-mail.

SMTP Server: Another term for SMTP Server Software.

SMTP Server Software: Software that implements the SMTP server features.

SNA (Systems Network Architecture): IBM's proprietary networking model.

snail mail: Jargon referring to the postal service, which delivers paper letters in a matter of days, as compared to e-mail, which delivers e-mails in a matter of seconds.

Sneakernet: A somewhat sarcastic term used to refer to the process of not using a computer network, but instead walking back and forth between computers with disks or CDs, moving files manually and requiring comfortable sneakers!

Sniffer: The trade name of a particular vendor's network analysis tool. (Sniffer is a trademark of the Network Associates Corporation.)

Source Address field: The 6-byte-long portion of an Ethernet header in which a NIC places its own Ethernet address before sending a frame.

source IP address: A field in the IP header that lists the IP address of the sender of the packet.

speaker: A device that converts analog electrical signals into sounds.

static IP route: A routing table entry that was created by having a network engineer configure a router with the routing information.

STP (shielded twisted-pair): A type of cabling that holds twisted pairs of wires and for which extra shielding is added to the wires to reduce EM interference.

straight-through cable: An Ethernet LAN cable for which the wire in pin 1 of the connector on one end of the cable connects to pin 1 on the other end of the cable; the wire in pin 2 on one connector is connected to pin 2; and so on.

structured wiring: An approach to running cables in a building that results in less clutter, fewer times to install a cable under the floor or inside the ceiling, and added convenience when installing new devices.

subnet: A subdivision of a Class A, B, or C network.

subnet number: A dotted-decimal number that represents a single subnet. The subnet number has the same value as the subnet's individual hosts in the network and subnet parts of the number, and all binary 0s in the host part of the number.

subnetting: The process of subdividing an IP network into multiple subnets.

switch: Networking hardware and software that forwards the network traffic back and forth between the various devices on the network.

switch port: A physical socket on the side of a switch into which an Ethernet cable can be inserted.

switching table: See [MAC address table]

SYN bit: See [TCP SYN flag]

SYN flag: See [TCP SYN flag]

Systems Network Architecture (SNA): See [SNA]

T1 circuit: See [T/1 line]

T1 line: A 4-wire circuit that runs at 1.544 Mbps and is composed of 24 channels at 64 Kbps each, plus 8 Kbps of management overhead.

TACACS+: Cisco proprietary protocol used between routers and AAA servers to exchange AAA information.

TCP ACK flag: A single bit inside the TCP header that signals whether a segment is anything except the first segment in a new TCP connection.

TCP connection: An agreement between application processes on two computers to use TCP to send data. The connection is created by sending three overhead messages between the two computers, which initialize the values of the fields in the TCP header.

TCP connection establishment: A process by which two computers exchange three TCP segments to initialize TCP header fields, thereby allowing an application program on each computer to communicate with an application program on the other computer, using TCP.

TCP destination port: A field inside the TCP header that identifies the specific application program on the receiving computer that should ultimately receive the data in the segment.

TCP port: A number that identifies an application process on a computer. TCP uses a source port and a destination port so that the receiver of a TCP segment can look at the destination port number and know to which application to give the data.

TCP segment: A TCP header, along with encapsulated data, created by TCP on behalf of an application.

TCP source port: A field inside the TCP header that identifies the specific application program on the sending computer that created the data in the segment.

TCP SYN flag: A single bit inside the TCP header that signals whether a segment is either the first or second segment in a new TCP connection. The first segment in a new connection has the SYN flag set to binary 1, and no other TCP flags set.

TCP/IP (Transmission Control Protocol/Internet Protocol): A public networking model that has widespread acceptance. The creation and management of the standards included in TCP/IP are performed by the Internet Engineering Task Force (IETF).

TCP/IP host name: See [host name]

telco: Jargon for telephone company.

telco switch: Equipment owned by the telephone company and installed in a CO.

telephone company (telco): See [telco]

top-level domain: The last suffix in any TCP/IP host name, which either identifies the purpose of that domain (such as .com for commercial enterprises), or the country of origin for the organization.

trailer: A bunch of bits placed after data prior to transmission. The bits allow a protocol to have a place to include information along with the user data to accomplish the function of the protocol.

Transmission Control Protocol/Internet Protocol (TCP/IP): See [TCP/IP]

transmission speed: The rate at which bits are transmitted over a WAN link, either in bits per second, kilobits per second, megabits per second, or gigabits per second.

trunk: Either refers to a VLAN trunk or to an Ethernet segment between two switches.

twisted pair: A pair of wires that are twisted together to significantly reduce electromagnetic interference.

UDP (User Datagram Protocol): A TCP/IP transport layer protocol that provides basic transport services but does not provide error recovery.

unicast address: An Ethernet MAC address that represents a single NIC.

uniform resource locator (URL): See [URL]

unshielded twisted-pair (UTP): See [UTP]

upload: A file transfer in which the computer gives a file that it has to another computer.

URL (uniform resource locator): A string of characters that can be used to uniquely identify a particular web page. Also known as a web address.

User Datagram Protocol (UDP): See [UDP]

username: A string of characters that represents a user of the network.

UTP (unshielded twisted-pair): A type of cabling that holds twisted pairs of wires, but for which no specific shielding is added to the cable. UTP cables are the most popular type in today's LANs.

VC (virtual circuit): A logical concept that represents the path that frames travel between Frame Relay DTEs. VCs are particularly useful when comparing Frame Relay to leased physical circuits.

vendor consortium: An organization of vendors who have similar goals and who cooperate to make their products work together, typically while waiting on standards organizations to finish finalizing a standard. The Frame Relay Forum is one such consortium.

virtual circuit (VC): See [VC]

virtual LAN: A broadcast domain created by configuring a switch and telling it which ports should be considered to be in the same broadcast domain.

virtual private network (VPN): See [VPN]

virus: A file, typically a program, that is placed onto a computer system using methods that circumvent security devices, with the intent to cause problems.

VLAN trunk: An Ethernet segment between two switches, over which the switches add a VLAN header to frames before sending the frames. The VLAN trunking header identifies in which VLAN the frame resides.

VLAN trunking: The process of adding and removing VLAN trunking headers when using a VLAN trunk.

VLAN trunking header: The header added to an Ethernet frame before it is sent over a VLAN trunk.

voice circuit: The physical path that the telco creates to carry the analog voice signal to support a voice phone call.

volt: The unit of measurement of voltage.

voltage: A term referring to electrical potential or electromotive force. In general terms, it is the energy and power that flow in electricity.

VPN (virtual private network): VPNs allow users of a public network, such as the Internet, to protect their data through encryption, making their network connection virtually private.

VPN client software: Software installed on an end user's PC that performs the encryption and decryption required to create a VPN.

VPN concentrator: A device and software installed at a corporate site, inside the corporate network, that performs the encryption and decryption required to create a VPN.

wall plate: A rectangular plastic mold that is used to cover a hole in a wall. The wall plate has a plug in it, into which a networking cable can be connected. The hidden side of the wall plate also has a cable, which in turn connects to the rest of the network.

WAN (wide-area network): A network that by definition includes components that are relatively far apart, typically in different buildings, and almost always requiring connectivity through a telephone company.

WAN link: Term referring to a service that the phone company provides, through which a pair of devices can send and receive data to each other, with the data passing through the telco's network.

WAN switch: See [telco switch]

waveform: Refers to the graph of the electrical signal on a wire, over time. A square waveform is used for the transmission of digital information.

web address: A string of characters that can be used to uniquely identify a particular web page. Also known as a uniform resource locator (URL).

web browser: Software that resides on the computer that the end user uses. The browser requests a web page from a web server, and after the server responds, the web browser displays the information that a web server sends.

web client: Software, running on an end user computer, that provides a user interface to the end user to display the web pages that are stored on web servers.

web content: See [content]

web object: See [object]

web page: Term that describes the actual content transferred from the web server to a web browser. This typically refers to what is seen at one point in time in the web browser.

web server: Software, running on some server computer hardware, that allows web browsers to connect to it for the purpose of retrieving web pages.

website: Jargon that refers to the web pages and web server that compose the web content for a particular organization or individual. This term typically includes all the web pages built by the company or person who owns the website.

well-known port: A port number that a server purposefully uses for a particular application, so when client computers want to use that service, they know what destination port to put into the TCP destination port field.

wide-area network (WAN): See [WAN]

wiring: A thin, long, somewhat cylindrical piece of metal that is used to transmit data. The wire typically has a plastic coating to provide strength to the brittle wire.

wiring closet: Typically a small room (hence the word closet) where all cables from all the computers run.

wiring panel: See [patch panel]

World Wide Web (WWW): See [WWW]

WWW (World Wide Web): The combined set of all websites in the universe. Often, the term WWW is used to generally refer to the Internet, specifically in the context of all the websites available through the Internet.

zip code: Stands for zone improvement plan. It's a five-or nine-digit decimal number that the U.S. Postal Service uses for fast, easy, high-speed sorting of mail.