Address Classes; Subnet Part Of An Ip Address - 3Com LANPLEX 2500 EXTENDEDPLEX®2500 EXTENDED User Manual

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.