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 9-9 Example: Subnet mask 255.255.0.0 applied to class A network
.
Table 9-9
Example: Subnet mask 255.255.0.0 applied to class A network
Class A Network Address
Network 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
As illustrated in
Table 9-9 Example: Subnet mask 255.255.0.0 applied to class A network
, 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.