Subnet Addressing; Figure 1-2. Example Of Subnetting A Class B Address - NETGEAR RT338 Reference Manual

Reference Guide for the Model RT338 ISDN Router

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
Class B
Network
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.
Although the preceding example uses the entire third octet for a subnet address, note that you are
not restricted to octet boundaries in subnetting. To create more network numbers, you need only
shift some bits from the host address to the network address. For instance, to partition a Class C
network number (192.68.135.0) into two, you shift 1 bit from the host address to the network
address. The new netmask (or subnet mask) is 255.255.255.128. The first subnet has network
number 192.68.135.0 with hosts 192.68.135.1 to 129.68.135.126, and the second subnet has
network number 192.68.135.128 with hosts 192.68.135.129 to 192.68.135.254.
Note: The number 192.68.135.127 is not assigned because it is the broadcast address
of the first subnet. And 192.68.135.128 is not assigned because it is the network address
of the second subnet.
1-10
1-2.
Subnet Node
7262
Introduction