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Define an IP Subnet Mask
An IP network may be divided into smaller portions by a process called
sub-netting. A subnet is specified using high end bits of the host field
of the IP address for network addressing. This is done with a subnet
mask. Thus, the size of the subnet (i.e. The number of bits available for
subnet addressing) is the size of the subnet mask minus the length of
the network field of the IP address for that class (8, 16 or 24 bits for
classes A, B and C respectively). For example, a small company is
connected to the Internet, they are assigned a single class C IP network
address (199.169.100.0). This network address allows the company to
define up to 255 host addresses within their network. Their network
will be attached to the Internet with an IP router.
If this company decides to split their network into two LANs to reduce
the load on their network, the original IP network address may be sub-
netted into two or more smaller IP networks consisting of a smaller
number of host addresses in LAN. This allows each of the sites to be a
smaller IP network and to be routed together to allow inter-network
communication.
The router allows masks from 8 to 32 bits. The mask size determines
how many bits of the host field of the original IP network address will
be used for the creation of subnets. In this example, a subnet mask
size of 26 will produce a subnet size of 2 bits (24 bits from the class C
network address field plus 2 bits from the host address field). Two bits
gives 4 possible sub-network addresses from the original IP network
address. Two of the resulting sub-networks will have either all zeros or
all ones as the subnet address; under standard subnets, these addresses
are reserved for network functions and hence are invalid addresses. So
setting a mask of 26 will generate two resulting sub-networks with up
to 62 host addresses each (64 potential addresses minus the all zero and
all one addresses). The new IP network addresses will be:
199.169.100.64 and 199.169.100.128. The subnet mask for the newly
created networks will be 255.255.255.192..