User`s guide

Glossary

134 E5810B User’s Guide

Subnet

Subnet is a portion of a network that shares a common

address component. On TCP/IP networks, subnets are

defined as all devices whose IP addresses have the same

prefix. For example, all devices with IP addresses that start

with 100.100.100. would be part of the same subnet. Dividing

a network into subnets is useful for both security and

performance reasons. IP networks are divided using a subnet

mask.

Subnet mask

Subnet mask is a mask used to determine what subnet an IP

address belongs to. An IP address has two components — the

network address and the host address. For example,

consider that the IP address is 150.215.017.009. Assuming

this is part of a Class B network, the first two numbers,

150.215 represent the Class B network address and the

second two numbers, 017.009 identify a particular host on

this network.

Subnetting enables a system administrator to further divide

the host part of the address into two or more subnets. In

this case, a part of the host address is reserved to identify

the particular subnet.

For example, the full address for 150.215.017.009 is

10010110.11010111.00010001.00001001. The Class B network

part is 10010110.11010111 and the host address is

00010001.00001001. If this network is divided into 14

subnets, the first four bits of the host address, 0001 are

reserved for identifying the subnet.

The subnet mask is the network address plus the bits

reserved for identifying the subnetwork. By convention, the

bits for the network address are all set to 1, though it would

also work if the bits were set exactly as in the network

address. In this case, the subnet mask is

11111111.11111111.11110000.00000000.