Technical data

TCP/IP Network Basics 7
Agilent Connectivity Guide 177
Subnets and Subnet Masks
As noted, an IP address consists of a network portion and a host portion.
A
subnet mask is a number that looks like an IP address that shows IP
how many bits are used for the network portion of the IP address by
“masking” the network portion of the IP address. Every IP address must
have a subnet mask. You can use standard or custom subnet masks.To
see how subnet masks are used, we will first define a
subnet.
What is a Subnet? Large networks can be divided by a System
administrator by creating a number of smaller networks, called
subnets.
The larger network is divided into subnets by using
routers (also called
default gateways
). All devices on one side of the router form a separate
subnet. Each subnet must have a unique set of IP addresses that are
defined by the
subnet mask for that network. Each side of the router
must have a separate network address.
Standard Subnet Masks A subnet mask (like an IP address) is 32
bits long, with several contiguous bits (all 1s) that represent the network
portion of the IP address and the rest of the bits (all 0s) that represent the
host portion of the IP address.
For example, the standard subnet mask for a Class A address is
255.0.0.0 (binary 1111 1111.0000 0000.0000 0000.0000 0000), so the
first eight bits (all 1s) are used for the network portion of the IP address
and the remaining 24 bits (all 0s) are used for the host portion of the IP
address. This table shows the standard subnet masks for Class A, Class
B, and Class C IP addresses.
Custom Subnet Masks You can create custom subnet masks for
Class A, Class B, and Class C IP addresses. For example, for a Class A
IP address, by using custom subnet mask 255.255.224.0 you can create
2,046 unique networks with 4,094 unique host on each network.
Class Standard Subnet
Mask
Standard Subnet Mask
(Binary Equivalent)
A 255.0.0.0 1111 1111. 0000 0000. 0000 0000. 0000 0000
B 255.255.0.0 1111 1111. 1111 1111. 0000 0000. 0000 0000
C 255.255.255.0 1111 1111. 1111 1111. 1111 1111. 0000 0000