NWW
TCP/IP Overview 248
Subnets
When an IP network address for a particular network class is
assigned to an organization, no provision is made for more than
one network being present at that location. Local network
administrators use subnets to partition a network into several
different subnetworks. Splitting a network into subnets can result
in better performance and improved use of limited network
address space.
Subnet Mask
The subnet mask is a mechanism used to divide a single IP network
into several different subnetworks. For a given network class, a
portion of an IP address that would normally be used to identify a
node is used, instead, to identify a subnetwork. A subnet mask
is applied to each IP address to specify the portion used for
subnetworks, and the portion used to identify the node.
For example, see
Table A.3
.
As illustrated in
Table A.3
, the Class A IP network address “15”
has been assigned to company ABC. To allow additional networks
at company ABC’s site, the subnet mask of 255.255.0.0 is used. This
subnet mask specifies that the second byte of the IP address will be
used to identify up to 254 subnets. Using this designation, each
device is uniquely identified on its own subnet, but company ABC
can incorporate up to 254 subnetworks without violating their
assigned address space.
Table A.3
Example: Subnet Mask 255.255.0.0 Applied to Class A
Network
Class A Network Address
15
xxx
xxx
xxx
Subnet Mask
255
255
0
0
IP Address fields with Subnet
Mask applied
Network
Subnet
Host
Host
Example of a node’s IP Address
on Subnet 1
15
1
25
7
Example of a node’s IP Address
on Subnet 254
15
254
64
2
