Subnetting Basics-1

A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.

The process of subnetting involves the separation of the network and subnet portion of an address from the host identifier. This is performed by a bitwise AND operation between the IP address and the (sub)network mask. The result yields the network address or prefix, and the remainder is the host identifier.

Determining the network prefix

An IPv4 network mask consists of 32 bits, a sequence of ones (1) followed by a block of 0s. The trailing block of zeros (0) designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address (192.168.5.130) and its associated /24 network mask (255.255.255.0). The operation is visualized in a table using binary address formats.

Determining the network prefix

An IPv4 network mask consists of 32 bits, a sequence of ones (1) followed by a block of 0s. The trailing block of zeros (0) designates that part as being the host identifier.
The following example shows the separation of the network prefix and the host identifier from an address (192.168.5.130) and its associated /24 network mask (255.255.255.0). The operation is visualized in a table using binary address formats.

	Binary form	Dot-decimal notation
IP address	11000000.10101000.00000101.10000010	192.168.5.130
Subnet mask	11111111.11111111.11111111.00000000	255.255.255.0
Network prefix	11000000.10101000.00000101.00000000	192.168.5.0
Host part	00000000.00000000.00000000.10000010	0.0.0.130

The mathematical operation for calculating the network prefix is the bitwise AND of IP address and subnet mask. The result of the operation yields the network prefix 192.168.5.0 and the host number 130.

Subnetting

Subnetting is the process of designating some high-order bits from the host part and grouping them with the network mask to form the subnet mask. This divides a network into smaller subnets. The following diagram modifies the example by moving 2 bits from the host part to the subnet mask to form four smaller subnets one quarter the previous size:

	Binary form	Dot-decimal notation
IP address	11000000.10101000.00000101.10000010	192.168.5.130
Subnet mask	11111111.11111111.11111111.11000000	255.255.255.192
Network prefix	11000000.10101000.00000101.10000000	192.168.5.128
Host part	00000000.00000000.00000000.00000010	0.0.0.2

	Prefix size Network mask Available subnets Usable hosts per subnet Total usable hosts /24 255.255.255.0 1 254 254 /25 255.255.255.128 2 126 252 /26 255.255.255.192 4 62 248 /27 255.255.255.224 8 30 240 /28 255.255.255.240 16 14 224 /29 255.255.255.248 32 6 192 /30 255.255.255.252 64 2 128 /31 255.255.255.254 128 2 * 256