Quick Reference Guide

Reference Guide for the Model RT338 ISDN Router
1-10 Introduction
Subnet Addressing
By looking at the addressing structures, you can see that even with a Class C address there are a
large number of hosts per network. If each end of a routed link requires a different network
number, such a structure is an inefficient use of addresses. It is unlikely that smaller office LANs
require that many devices. You can resolve this problem by using a technique known as subnet
addressing.
Subnet addressing lets you split one IP network address into smaller multiple physical networks,
known as subnetworks. Some of the node numbers are used as a subnet number. A Class B address
provides 16 bits of node numbers, translating to 64,000 nodes. Most organizations do not use
64,000 nodes; free bits can be reassigned. Subnet addressing makes use of those bits that are free,
as illustrated in Figure 1-2.
Figure 1-2. Example of Subnetting a Class B Address
A Class B address can be effectively translated into multiple Class C addresses. For example, the
IP address of 172.16.0.0 is assigned, but node addresses are limited to 255 maximum, allowing
8 extra bits to use as a subnet address. The IP address of 172.16.97.235 would be interpreted as
IP network address 172.16, subnet number 97, and node number 235. In addition to extending
the number of addresses available, subnet addressing provides other benefits. Subnet addressing
lets a network manager construct an address scheme for the network by using different subnets for
other geographical locations in the network or for other departments in the organization.
7262
Class B
Network Subnet Node