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.
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| Fig 1.1- Sample Class C Subnetting |
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 |
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| 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 |
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