Reference Guide for the Model RT210/RT211 ISDN Routers
TCP/IP Routing B-3
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. Such a structure is an inefficient use of addresses if each end of
a routed link requires a different network number. It is unlikely that the smaller office LANs would
have that many devices. You can resolve this problem by using a technique known as subnet
addressing.
Subnet addressing allows us to split one IP network address into smaller multiple physical
networks known as subnetworks. Some of the node numbers are used as a subnet number instead.
A Class B address gives us 16 bits of node numbers translating to 64,000 nodes. Most
organizations do not use 64,000 nodes, so there are free bits that can be reassigned. Subnet
addressing makes use of those bits that are free.
Figure B-2. Example of Class B address
A Class B address can be effectively translated into multiple Class C addresses. Example: The IP
address of 129.46.0.0 is assigned, limiting node addresses to 255 maximum, allowing 8 extra bits
to use as a subnet address. The IP address of 129.46.97.235 would be interpreted as IP address
129.46, subnet number 97, and node number 235. As well as extending the number of addresses
available, subnet addressing allows a network manager to construct an address scheme for his or
her network by using different subnets for different geographical locations in the network or for
different departments in the organization.
7262
Class B
Network Subnet Node
NGRRT210.BK Page 3 Friday, October 17, 1997 2:37 PM