3Com 2500 manual Address Classes

Elements of IP Routing 4-3

Address Classes

The boundary of the network part and the host part depends on the class that the central agency assigns to your network. The primary classes of IP addresses are Class A, Class B, and Class C.

■Class A addresses — have 8 bits for the network part and 24 bits for the host part. Although only a few Class A networks can be created, each can contain a very large number of hosts.

■Class B addresses — have 16 bits for the network part and 16 bits for the host part.

■Class C addresses — have 24 bits for the network part and eight bits for the host part. Each Class C network can contain only up to 254 hosts, but many such networks can be created.

The class of an IP address is designated in the high-order bits of the network parts of the address.

Subnet Part of an IP Address

In some environments, the IP address contains a subnet part. Subnetting allows a single Class A, B, or C network to be further subdivided internally while still appearing as a single network to other networks. The subnet part of the IP address is only visible to those hosts and gateways on the subnet network.

When an IP address contains a subnet part, a subnet mask is used to identify which bits are the subnet address and which are the host address. A subnet mask is a 32-bit number that uses the same format and representation as IP addresses. Each IP address bit corresponding to a 1 in the subnet mask is in the network or subnet part of the address. Each IP address bit corresponding to a 0 is in the host part of the IP address. See Figure 4-3.