Subnet Addressing - Bay Networks Nautica 200 Reference Manual

Understanding the Technology
Class A addresses use an 8-bit network number and a 24-bit node
number. Class A addresses are in the range 001.xxx.xxx.xxx to
127.xxx.xxx.xxx (addresses 127.xxx.xxx.xxx are reserved and
cannot be assigned to networks or hosts).
Class B addresses use a 16-bit network number and a 16-bit node
number. Class B addresses are in the range 128.001.xxx.xxx to
191.254.xxx.xxx.
Class C addresses can have up to 254 hosts on a network. They
use a 24-bit network number and an 8-bit node number. Class C
addresses are in the range 192.000.000.xxx to 223.255.255.xxx.
Class D addresses are used for multicasts (messages sent to many
hosts). Class D addresses are in the range 224.000.000.001 to
239.255.255.254.
Class E addresses are for experimental use.
This addressing structure allows IP to uniquely identify each
physical network and each node on each physical network.

Subnet Addressing

Even with a Class C address, there is a large number of hosts per
network. This is an inefficient use of addresses if each end of a
routed link needs a different network number, because it is
unlikely that the smaller-office LANs would have that many
devices. This can be resolved by using subnet addressing.
Subnet addressing splits one IP network address into smaller
multiple physical networks (subnetworks). Part of the node
number is used as the subnet number instead. For example, the
16-bit node number of a Class B address translates to 64,000
nodes. Most organizations do not use 64,000 nodes, so there are
bits free to be used for subnet addressing, as shown below.
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