Assigning IP Addresses
| Having established the need for each system on a TCP/IP based network to have a unique address, we can now go on to look at how those systems receive their addresses. Static AddressingStatic addressing refers to the manual assignment of IP addresses to a system. There are two main problems with this approach. Statically configuring one system with the correct address is simple, but in the course of configuring, say, a few hundred systems, mistakes are likely to be made. If the IP addresses are entered incorrectly, the system will most likely not be capable of connecting to other systems on the network. Another drawback of static addressing is reconfiguration. If the IP addressing scheme for the organization changes, each system must again be manually reconfigured. In a large organization with hundreds or thousands of systems, such a reconfiguration could take a considerable amount of time. These drawbacks to static addressing are so significant that nearly all networks use dynamic IP addressing. Dynamic AddressingDynamic addressing refers to the assignment of IP addresses automatically. On modern networks the mechanism used to do this is the Dynamic Host Configuration Protocol (DHCP). DHCP is a protocol, part of the TCP/IP protocol suite, which enables a central system to provide client systems with IP addresses. Assigning addresses automatically with DHCP alleviates the burden of address configuration and reconfiguration that occurs with static IP addressing. The basic function of the DHCP service is to automatically assign IP addresses to client systems. To do this, ranges of IP addresses, known as scopes, are defined on a system that is running a DHCP server application. When another system configured as a DHCP client is initialized, it asks the server for an address. If all things are as they should be, the server assigns an address to the client for a predetermined amount of time, which is known as the lease, from the scope. A DHCP server can typically be configured to assign more than just IP addresses; they are often used to assign the subnet mask, the default gateway, and Domain Name Service (DNS) information. Using DHCP means that administrators do not have to manually configure each client system with a TCP/IP address. This removes the common problems associated with statically assigned addresses such as human error. The potential problem of assigning duplicate IP addresses is also eliminated. DHCP also removes the need to reconfigure systems if they move from one subnet to another, or if you decide to make a wholesale change of the IP addressing structure.
Like DHCP, BOOTP is a broadcast-based system. Therefore, routers must be configured to forward BOOTP broadcasts. Today, it is far more likely that DHCP, rather than BOOTP, is used. APIPAAutomatic Private IP addressing (APIPA) is a feature introduced with Windows 98, and has been included in all subsequent Windows versions. The function of APIPA is that a system is capable of providing itself with an IP address in the event that it is incapable of receiving an address dynamically from a DHCP server. In such an event, APIPA assigns the system an address from the 169.254.0.0 address range and configures an appropriate subnet mask (255.255.0.0). However, it doesn't configure the system with a default gateway address. As a result, communication is limited to the local network.
The idea behind APIPA is that systems on a segment can communicate with each other in the event of DHCP server failure. In reality, the limited usability of APIPA makes it little more than a last resort measure. For example, imagine that a system is powered on while the DHCP server is operational and receives an IP address of 192.168.100.2. Then the DHCP server fails. Now, if the other systems on the segment are powered on and are unable to get an address from the DHCP server because it is down, they would self-assign addresses in the 169.254.0.0 address range via APIPA. The systems with APIPA addresses would be able to talk to each other, but they couldn't talk to a system that received an address from the DHCP server. Likewise, any system that received an IP address via DHCP would be unable to talk to systems with APIPA assigned addresses. This, and the absence of a default gateway, is why APIPA is of limited use in real-world environments. TCP/IP ProtocolsThe TCP/IP protocol suite is made up of many different protocols, each of which performs a specific task or function. CompTIA identifies a number of these protocols in the objectives for the Network+ exam. The following sections look at the functions of these protocols and their purposes. Internet Protocol (IP)The IP protocol is a network layer protocol responsible for transporting data between network devices and for handling IP addressing. IP is a connectionless protocol, meaning that data delivery is not guaranteed; it takes the best-effort approach. Transmission Control Protocol (TCP)TCP functions at the transport layer of the OSI model and is a connection-oriented protocol that uses IP as its network protocol. Being connection-oriented means that TCP establishes a mutually acknowledged session between two hosts before communication takes place. TCP provides reliability to IP communications. Specifically, TCP adds features such as flow control, sequencing, and error detection and correction. For this reason, higher-level applications that need guaranteed delivery use TCP rather than its lightweight and connectionless brethren, the User Datagram Protocol (UDP). User Datagram Protocol (UDP)UDP operates at the transport layer of the OSI model and performs functions similar to that of TCP, with one notable difference; UDP is a connectionless protocol and does not guarantee data delivery. Both TCP and UDP use IP as its transport protocol. Because UDP does not need to guarantee data delivery it is much more efficient than TCP, so for applications that don't need the added features of TCP, UDP is much more economical in terms of bandwidth and processing effort. A good example of UDP is an online radio station that sends data but does not confirm data delivery. File Transfer Protocol (FTP)The FTP protocol is an application layer protocol that provides a method for uploading and downloading files from a remote system running FTP server software. FTP uses the TCP transport protocol to guarantee the delivery of data packets. FTP has some basic security capabilities, such as a capability to authenticate users. However, rather than create a user account for every user, you can configure FTP server software to accept anonymous logons. When you do this, the username is anonymous, and the password is normally the user's email address. Most FTP servers that offer files to the general public operate in this way. FTP is popular for distributing files over the Internet but is also used within organizations that need to frequently exchange large files with other people or organizations that find it impractical to use regular email.
FTP is platform independent, meaning that all the common network operating systems offer FTP server capabilities. In addition, all commonly used client operating systems offer FTP client functionality. Alternatively, third-party utilities such as SmartFTP and CuteFTP are often used. There are several commands that can be used with FTP; you are expected to understand these commands for the Network+ exam. Table 5.5 lists the commands that are used with the FTP protocol.
Secure File Transfer Protocol (SFTP)One of the big problems associated with FTP is that it transmits data between sender and receiver in an unencrypted format. The solution is the Secure File Transfer Protocol, which is based on Secure Shell (SSH) technology. SSH provides robust authentication between sender and receiver, in addition to encryption capabilities. SFTP is implemented through client and server software that is available for all commonly used computing platforms. Trivial File Transfer Protocol (TFTP)A variation on FTP is TFTP, which is also a file transfer mechanism. FTP and TFTP are both application layer protocols; however, TFTP does not have the rudimentary security capability or the level of functionality that FTP has. TFTP uses only UDP as a transport protocol, making it a connectionless protocol. As such, it has a lower overhead than FTP.
Another feature that TFTP does not offer is directory navigation. In FTP, commands can be executed to navigate around and manage the file system; TFTP offers no such capability. TFTP requires that you request not only exactly what you want, but also from what specific location. Simple Mail Transfer Protocol (SMTP)The SMTP protocol defines how mail messages are sent between hosts. SMTP is a connection-oriented protocol; it uses TCP connections to guarantee error-free delivery of messages. SMTP is not overly sophisticated and requires that the destination host always be available. For this reason, mail systems spool incoming mail so that users can read it at a later time. How the user then reads the mail depends on how the client accesses the SMTP server. SMTP is an application layer protocol. Today, SMTP is often used to send email between servers, whereas another protocol such as POP3 or IMAP4 is used to download the email from the server to a client system. Hypertext Transfer Protocol (HTTP)In practical uses, HTTP is the protocol that allows text, graphics, multimedia, and other material to be downloaded from an HTTP server (commonly called a Web server). HTTP defines which actions clients can request and how servers should answer those requests. HTTP uses TCP as a transport protocol, making it a connection-oriented protocol. However, it can also use UDP for certain functions. HTTP uses a uniform resource locator (URL) to determine which page should be downloaded from the remote server. The URL contains the type of request (for example, http://), the name of the server being contacted (for example, www.novell.com), and optionally the page being requested (for example, /support). The result is the syntax that Internet-savvy people are familiar with: http://www.novell.com/support. HTTP functions at the application layer of the OSI model. Hypertext Transfer Protocol Secure (HTTPS)Normal HTTP requests are sent in clear text, and for some Internet transactions such as online banking or e-commerce, this poses a significant security problem. The solution for such applications is to use the HTTPS protocol. HTTPS uses a security technology known as Secure Sockets Layer (SSL), which encrypts the information sent between the client and the host. You can tell when you are accessing a page with HTTPS because the URL will have an HTTPS:// address as opposed to 'plain' HTTP, which uses an address of HTTP://. An example of an HTTPS URL address is https://www.nationalonlinebank.com. Like HTTP, HTTPS uses the TCP transport protocol and operates at the application layer of the OSI model. Post Office Protocol version 3 (POP3) and Internet Message Access Protocol version 4 (IMAP4)Both POP3 and IMAP4 are mechanisms for downloading, or pulling, email from a mail server. They are necessary because, although the mail is transported around the network via SMTP, users cannot always read it immediately so it must be stored in a central location. From this location, it must then be downloaded, which is what POP3 and IMAP4 allow you to do. One of the problems with POP3 is that the password used to access a mailbox is transmitted across the network in clear text. That means if someone wanted to, he could determine your POP3 password with relative ease. This is an area in which IMAP4 offers an advantage over POP3. It uses a more sophisticated authentication system, which makes it harder for someone to determine a password. TelnetThe function of Telnet is to allow the establishment of sessions on a remote host. A user can then execute commands on that remote host as if he were physically sitting at the system. Telnet is widely used to access UNIX and Linux systems, as well as to administer some managed networking equipment such as switches or routers. Telnet uses TCP as a transport layer protocol and functions at the application layer of the OSI model.
Secure Shell (SSH)Secure Shell (SSH) is a secure alternative to Telnet. SSH provides security by encrypting data as it travels between systems. It also provides more robust authentication systems than Telnet. Although SSH, like Telnet, is primarily associated with UNIX and Linux systems, implementations of SSH are available for all commonly used computing platforms including Windows and Macintosh. As discussed earlier, SSH is the foundational technology for the Secure File Transfer Protocol (SFTP). Internet Control Message Protocol (ICMP)ICMP is a protocol that works with IP to provide error checking and reporting functionality. In effect, ICMP is a tool that IP uses in its quest to provide best-effort delivery. ICMP functions at the network layer of the OSI model. ICMP can be used for a number of functions. Its most common is probably the widely used and incredibly useful ping utility. ping sends a stream of ICMP echo requests to a remote host. If the host is able to respond, it does so by sending echo reply messages back to the sending host. In that one simple process, ICMP enables the verification of the protocol suite configuration of both the sending and receiving nodes and any intermediate networking devices. Address Resolution Protocol/Reverse Address Resolution Protocol (ARP/RARP)The basic function of the ARP protocol is to resolve IP addresses to Media Access Control (MAC) addresses. When a system attempts to contact another host, IP first determines whether the other host is on the same network it is on by looking at the IP address. If IP determines that the destination is on the local network, it consults the ARP cache to determine whether it has a corresponding entry.
If there is not an entry for the host in the ARP cache, IP sends a broadcast on the local network, asking the host with the target IP address to send back its MAC address. The communication is sent as a broadcast because without the target system's MAC address, the source system is unable to communicate directly with the target system. The Reverse Address Resolution Protocol (RARP) performs the same function as ARP, but in reverse. In other words, it resolves MAC addresses to IP addresses. RARP makes it possible for applications or systems to learn their own IP address from a router or DNS server. Such a resolution comes in handy for tasks such as performing reverse lookups in DNS. Network Time Protocol (NTP)NTP uses the TCP transport protocol and is the protocol that facilitates the communication of time information between systems. The idea is that one system configured as a time provider transmits time information to other systems that can be both the time receivers and the time providers to other systems. Network News Transport Protocol (NNTP)The Network News Transfer Protocol (NNTP) is a protocol associated with posting and retrieving messages from newsgroups. A newsgroup is the name given to a discussion forum that is hosted on a remote system. By using NNTP client software, similar to that included with many common email clients, users can post, reply, and retrieve messages. NNTP is an application layer protocol that uses TCP as its transport mechanism. Secure Copy Protocol (SCP)The Secure Copy Protocol (SCP) is another protocol based on Secure Shell (SSH) technology. SCP provides a secure means to copy files between systems on a network. By using SSH technology, it encrypts data as it travels across the network, thereby securing it from eavesdropping. It is intended as a more secure substitute for the Remote Copy Protocol (RCP). SCP is most commonly associated with UNIX or Linux platforms, though it is available as a command-line utility or as part of application software for most commonly used computing platforms. SCP operates at the application layer of the OSI model. Lightweight Directory Access Protocol (LDAP)The Lightweight Directory Access Protocol (LDAP) is a protocol that provides a mechanism to access and query directory services systems. In the context of the Network+ exam, these directory services systems are most likely to be Novell Directory Services (NDS) and Microsoft's Active Directory. Although LDAP supports command-line queries that are executed directly against the directory database, most LDAP interactions will be via utilities such as an authentication program (network logon) or locating a resource in the directory through a search utility. LDAP operates at the application layer of the OSI model. Internet Group Management Protocol (IGMP)The Internet Group Management Protocol (IGMP) protocol is associated with the process of multicasting. Multicasting is a mechanism by which groups of network devices can send and receive data between the members of the group at one time, rather than separately sending messages to each device in the group. The IGMP protocol is used to register devices into a multicast group, as well as to discover what other devices on the network are members of the same multicast group. Common applications for multicasting include groups of routers on an internetwork and videoconferencing clients. IGMP operates at the network layer of the OSI model. Line Printer Remote (LPR)The Line Printer Remote (LPR) protocol provides a means to connect to print servers on a network. It is a generic printing protocol supported by all commonly used operating systems including UNIX, Windows, and Linux. To make use of LPR, client software is installed on a system. When a file is sent to print, it is channeled over the network by LPR to a print server or printer. That server or printer runs a print server program, normally the Line Printer Daemon (LPD), which accepts the LPR information and adds that job to the print queue. LPR operates at the application layer of the OSI model. TCP/IP Protocol Suite SummaryThe details of each of the protocols discussed in the preceding sections are summarized in Table 5.6. You can use this table for review before you take the Network+ exam.
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