Subnet Addressing - Bay Networks NauticaRS Reference Manual

Introduction to Protocols and Routing

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.
In this way a Class B address can be translated into multiple Class
C addresses, which extends the number of addresses available.
Example: assigning an IP address of 129.46.0.0 with a maximum
of 253 node addresses leaves 8 bits free for use as a subnet
address, so the IP address 126.46.97.235 is interpreted as IP
address 129.46, subnet number 97 and node number 235.
Subnet addressing also allows network managers to construct
address schemes for their networks by using different subnets for
different geographical locations, or for different departments in
the organization.
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