Subnet mask

An IP address is divided into two parts: network and host parts. For example, an IP class A address consists of 8 bits identifying the network and 24 bits identifying the host. This is because the default subnet mask for a class A IP address is 8 bits long. (or, written in dotted decimal notation, 255.0.0.0). What does it mean? Well, like an IP address, a subnet mask also consists of 32 bits. Computers use it to determine the network part and the host part of an address. The 1s in the subnet mask represent a network part, the 0s a host part.

Computers works only with bits. The math used to determine a network range is binary AND.

binary and

Let’s say that we have the IP address of 10.0.0.1 with the default subnet mask of 8 bits (255.0.0.0).
First, we need to convert the IP address to binary:

IP address: 10.0.0.1 = 00001010.00000000.00000000.00000001
Subnet mask 255.0.0.0 = 11111111.00000000.00000000.0000000

Computers then use the AND operation to determine the network number:

determining the network number

The computer can then determine the size of the network. Only IP addresses that begins with 10 will be in the same network. So, in this case, the range of addresses in this network is 10.0.0.0 – 10.255.255.255.

NOTE
A subnet mask must always be a series of 1s followed by a series of 0s.

 

Slash Notation

Aside from the dotted decimal format, we can also write the subnet mask in slash notation. It is a slash ‘/’ then followed by the subnet mask bits. To determine the slash notation of the subnet mask, convert the dotted decimal format into binary, count the series of 1s, and add a slash on the start.

For example, we have the dotted decimal subnet mask of 255.0.0.0. In binary, it is 11111111.00000000.00000000.0000000. The number of succeeding 1s are 8, therefore the slash notation of 255.0.0.0 is /8.


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