Network Addressing

Network addresses identify devices that are attached to an internetwork, either as individual devices or as members of a group. Three types of addressing are used in internetworks:

  • Data-link layer addresses

  • Media Access Control (MAC) addresses

  • Network layer addresses

Each of these is discussed in the following sections.

Data-Link Layer Addresses

Data-link layer addresses identify each network device's physical network connection. Data-link addresses are referred to as physical or hardware addresses and exist within a flat address space, having a pre-established and fixed relationship to a specific device.

End systems often have one physical network connection and thus have only one data-link address, whereas routers and other internetworking devices can have multiple physical network connections, and therefore have multiple data-link addresses. This physical address is known as the MAC, address.

MAC Addresses

MAC addresses consist of a subset of data-link layer addresses. MAC addresses identify network entities in LANs that implement the IEEE MAC addresses of the data-link layer. As with most data-link addresses, MAC addresses are unique for each LAN interface. Figure 1-7 illustrates the relationship between MAC addresses, data-link addresses, and the IEEE sub-layers of the data-link layer.

Figure 1-7. MAC Addresses, Data-Link Addresses, and the IEEE Sub-Layers

graphics/01fig07.gif

MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits (0-9, A-F). The MAC address is broken down as follows:

  • First six hexadecimal digits Administered by the IEEE. Identifies the manufacturer or vendor and comprises the Organizationally Unique Identifier (OUI).

  • Last six hexadecimal digits Administered by the specific vendor and made up of the interface serial number.

MAC addresses are also known as burned-in addresses (BIAs) because the address is burned into read-only memory (ROM) and copied into random-access memory (RAM) when the interface card is initialized (powered up). Figure 1-8 illustrates the MAC address format.

Figure 1-8. MAC Address (48 Bits)

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The following is an example of a MAC address:

 00-02-3b-00-b1-5f 

In this case, the OUI is 00-02-3b and the vendor assigned code is 00-b1-5f.

Network Layer Addresses

A network layer address identifies an internetwork device at the network layer of the OSI Model. Network addresses exist within a hierarchical address space and are often called virtual or logical addresses.

End systems require one network layer address for each network layer protocol they support. The network layer protocol in turn is defined by the internetwork to which the network device is attached. For example, suppose that on a router with three interfaces, one interface is attached to an AppleTalk network, another interface is attached to a Novell network, and the third interface is attached to an IP network. The router must have network layer addresses for each network; in this case, an AppleTalk, IPX (Novell), and an IP network address.

Because internetworks use network addresses to route traffic through the network, there is a need to map network addresses to MAC addresses. When the network layer has determined the destination host's network address, the sending device must forward the information over a physical network using a MAC address. Different protocol suites use different methods to perform this mapping of MAC-to-IP addressing, with the most popular being the Address Resolution Protocol (ARP).

Hierarchical Versus Flat Address Space

Internetwork address space takes one of two forms:

  • Hierarchical address space is organized into subgroups, or subnetworks (subnets), each successively narrowing an address until it points to a single device (in a manner similar to street addresses). Telephone numbers are an example of hierarchical address space. For example, the North American Numbering Plan (NANP) users the following hierarchy: NPA-NXX-xxxx; where NPA is the area code and NXX is the central office (CO) exchange responsible for the "xxxx" extensions.

  • Flat address space is organized into a single group. An example of flat address space would be the U.S. Social Security Number; no two Social Security numbers can be logically grouped together.

Hierarchical addressing offers advantages over flat-addressing schemes in that addresses can be grouped together to form a single, larger address designation. For example, if a telephone number has the area code of 202, it signifies that the number is part of the Washington, DC telephone exchange group.

Technical Note: ARP

ARP is the method used in the TCP/IP suite. When a network device needs to send data to another device on the same network, the sending device knows the source and destination network addresses for the data transfer, but must map the destination network address to a MAC address before forwarding the data.

Before sending data across the network, the sending host will check its ARP table to see whether it has already discovered the destination host's MAC address. If the sending host has not, it will send a broadcast on the network with the destination host's IP address contained in the broadcast. Every station on the network receives the broadcast and compares the received IP address to its own. Only the receiving host with the matching IP address replies to the sending host with a packet containing the MAC address for the station. The sending host then adds the learned MAC address to its ARP table for future reference and then begins to send the data.

Network Address Assignments

Network addresses are assigned to device interfaces in one of two ways:

  • Static Addresses are assigned by a network administrator according to an internetwork addressing plan and do not change without a network administrator manually changing the address.

  • Dynamic Addresses are obtained by network devices when the devices are attached to a network, by means of some protocol-specific process, such as DHCP (Dynamic Host Configuration Protocol). A device using a dynamic address is often given a different address each time the device connects to the network, usually assigned by a dynamic address pool (maintained by a dynamic address server). Server-assigned network addresses are recycled for reuse by other network devices when the device holding the address disconnects from the network.

Network Addresses Versus Names

Internetwork devices often have both a name and a network address associated with them. Internetwork names tend to be location-independent and remain associated with a device wherever that device might be located. For example, if a network device is relocated from one building or floor to another the device name does not change. Internetwork addresses often are location-dependent and change when a device is moved. MAC addresses are an exception to this rule because of their nature (MAC addresses are BIAs, embedded in the device's network card). As network addresses are mapped to MAC addresses, names are mapped to network addresses.

The Internet uses the Domain Name System (DNS) to map the name of a device to its IP address. For example, it is easier for users to remember www.cisco.com than the IP address 198.133.219.25. Therefore, when a user types www.cisco.com in a web browser, the user's computer (host) performs a DNS lookup of the IP address for Cisco's web server and then communicates with the web server using the learned network address.

Technical Note: DNS

The group of computers listed in a single DNS naming scheme is called a zone. This list is a single definitive list of DNS names and associated IP addresses. A zone might be a top-level national domain, such as .com (commercial) or .gov (government). DNS service for subsidiary zones can be delegated within that zone, along with any subsidiary domains.

The DNS system is its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another DNS server, and so on, until the correct IP address is returned.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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