IP Addressing

Of the different addressing schemes, IP addressing is the most important to understand because you must conceptually comprehend how these devices communicate to effectively build networks on top of an IP infrastructure.

Many protocols exist, and each has a different addressing scheme.

Network layer addressing is normally hierarchical. As compared to the Public Switched Telephone Network (PSTN) in the North American Numbering Plan Association (NANPA) network of today, each Numbering Plan Area (NPA) includes a region, with a prefix (Nxx) denoting a sub-region and station identifier (xxxx) denoting the actual phone.

Network layer addressing lies at Layer 3 of the OSI model. This enables a group of computers to be given similar logical addresses. Logical addressing is similar to determining a person's address by looking at his or her country, state, ZIP code, city, and street address.

Routers forward traffic based on the Layer 3 or network layer address. IP addressing supports five network classes. The bits at the far left indicate the network class, as follows:

• Class A networks are intended mainly for use with a few large networks because they provide only seven bits for the network address field.

• Class B networks allocate 14 bits for the network address field and 16 bits for the host address field. This address class offers a good compromise between network and host address space.

• Class C networks allocate 21 bits for the network address field. They provide only 8 bits for the host field, however, so the number of hosts per network can be a limiting factor.

• Class D addresses are reserved for multicast groups, as described formally in RFC 1112. In class D addresses, the four highest-order bits are set to 1, 1, 1, and 0.

• Class E addresses also are defined by IP but are reserved for future use. In class E addresses, the four highest-order bits are set to 1, and the fifth bit is always 0.

IP addresses are written in dotted decimal format—for example, Figure 7-2 shows the address formats for class A, B, and C IP networks. An easy way to think of class addressing is that the more networks you have, the fewer hosts you can have on that network.

Figure 7-2. Class A, B, and C Address Formats

Figure 7-2. Class A, B, and C Address Formats

You can also divide IP networks into smaller units called subnets. Subnets provide extra flexibility for network administrators. Assume, for example, that a network is assigned a class B address, and all the nodes on the network currently conform to a class B address format. Then assume that the dotted decimal representation of this network's address is (all 0s in the host field of an address specify the entire network).

Rather than change all the addresses to some other basic network number, the administrator can subdivide the network using subnetting. He can do this by borrowing bits from the host portion of the address and using them as a subnet field, as shown in Figure 7-3.

Figure 7-3. Subnetting a Class B Address

Figure 7-3. Subnetting a Class B Address

Although this section discusses the makeup of IP addressing, it does not explain how a router knows where to send an IP packet. This is discussed in the next section.

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