The Binary Subnet Mask

A subnet mask is a pattern of bits that is usually used to isolate specific portions of a particular Internet Protocol address. When the mask is applied to an IP address, the network interface's network routing prefix and host address are produced. The routing prefix is known by names such as subnetwork or subnet. The masks are used by network hardware and software to manage and isolate a certain part of the network.

A routing prefix is generally expressed in Classless Inter-Domain Routing (CIDR) notation. This includes following the notations with a slash and the number of the significant parts in the prefix. When a subnet mask is logically calculated with a network interface's IP address, the result is usually the starting portion of the subnet's address, that particular interface is belonging to. The remaining part of the IP network address is the interface's identifier of the host. An Internet Protocol version 4 or IPv4 mask is usually expressed by four octets in dotted decimal form.

The mask is, represented in dotted decimal notation, which is similar to the IP. The commonly used binary code is the one which uses the dual representation of 11111111 11111111 11111111 0000. Another commonly used presentation is 255.255.255.0. This can be used to determine the address of the user's IP. There are different ways of calculating subnet mask, some of which are listed as follows:

1)The network class (A,B,C) needs to be determined based on IP address:
- If the address begin with 1 to 126, it is Class A
- If the address begin with 128 to 191, it is Class B
- If the address begins with 192 to 223 it is Class C

2)The number of bits need to be determined to define subnets:
- Number of subnets = (2^Number of bits)-2
- Number of bits= Log2 (Number of subnets 2)
For instance- Number of bits = Log2 (6 2)= Log2(8)=3. The three bits in the IP subnet is used as a subnet portion

3)The mask is composed of subnet mask in binary form by extending the default subnet mask with subnet bits. The default set subnet mask for classes A to C are:
11111111.00000000.00000000.00000000 (Class A, network part 8)
11111111.11111111.00000000.00000000 (Class B, network part 16)
11111111.11111111.11111111.00000000 (Class C, network part 24)

4)The binary subnet mask is converted to the decimal-dot form. The binary form contains four octets (8 bits in each). Use the following rules:
For "1111111" octet, write "255"
For "00000000" octet write "0"
If the octet contains both "1" and "0" the following formula needs to be used:
Integer number= (128 x n) (64 x n) (32 x n) (16 x n) (8 x n) (4 x n) (2 x n) (1 x n)
(where can either be "1" or "0")

There are various purpose of this feature. It is an important part of subnetting. A user's PC may be unable to access the network and host portions without it. It also helps to recognize the protocol ID. It helps in functions like decreasing traffic to a particular website, and identify terminals that require to be connected. For the subnet to work, it is essential to establish network connection. This helps the device to know, if the subnet is connected to the Internet. The default route or the address is found by the equipment, with the help of the local subnet or remote network.

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