Subnet Mask

What is Subnet Mask?

An IP address has two components, the network address and the host address. A subnet mask separates the IP address into the network and host addresses (<network><host>). Subnetting further divides the host part of an IP address into a subnet and host address (<network><subnet><host>) if additional subnetwork is needed. It is called a subnet mask because it is used to identify network address of an IP address by perfoming a bitwise AND operation on the netmask.

A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address. Subnet Mask is made by setting network bits to all "1"s and setting host bits to all "0"s. Within a given network, two host addresses are reserved for special purpose, and cannot be assigned to hosts. The "0" address is assigned a network address and "255" is assigned to a broadcast address, and they cannot be assigned to hosts

How to calculate Subnet Mask?

Consider IP address 192.168.2.0 is divided into 4 subnets i.e. 192.168.2.0, 192.168.2.64, 192.168.2.128, 192.168.2.192 networks. Now we have to calculate subnet mask for above four networks to identify each network or we can say to identify the destination network to which packet belongs. If we see our address it belongs to class C address. From class C network address first three octet is network id and last octet is host id.

So first step to calculate subnet mask is to make all network id bits to 1

11111111 . 11111111 . 11111111 . 00000000

Second Step:  For sub netting we have borrowed first two bits from last octet i.e. from host id. Make that two bits to 1. After making 1 address is

11111111 . 11111111 . 11111111 . 11000000

Now this your subnet mask for given address and in decimal form 255.255.255.192.

How to find an IP address belongs to which sub network?

Now we have seen how to find subnet mask but it is important to find how it works?. Now consider an IP address 192.168.2.75. To find it follow some steps

Step 1: Convert an IP address in binary

11000000 . 10101000 . 00000010 . 01001011

Step 2: Do Logical AND operation with subnet mask

11000000 . 10101000 . 00000010 . 01001011

11111111 . 11111111 . 11111111 . 11000000

 

11000000 . 10101000 . 00000010 . 01000000

i.e. 192.168.2.64 this is network address for 192.168.2.75