How to Subnet a Network — Step by Step
A practical guide to subnetting: dividing a network into smaller subnets, calculating network addresses, and choosing the right prefix length.
The subnetting process
Subnetting divides a larger network into smaller segments. Each subnet borrows bits from the host portion, increasing the prefix length and creating more, smaller networks.
If you need 4 subnets, you need at least 2 bits (2² = 4). For 6 subnets, you need 3 bits (2³ = 8, enough for 6).
If a subnet needs 50 hosts, you need at least 6 host bits (2⁶ = 64, giving 62 usable). That means a /26.
Your prefix must accommodate both requirements. Start with the parent block (e.g. /24) and extend it by the number of subnet bits.
The subnet size is 2^(32−prefix). For /26: 2^6 = 64. First subnet: .0, second: .64, third: .128, fourth: .192.
Make sure the subnets fit within the parent block and that each has enough hosts for your needs.
Example: Split 192.168.1.0/24 into 4 subnets
We borrow 2 bits from the host portion of a /24 to create 4 equal /26 subnets:
| Subnet | CIDR | Range | Usable Hosts |
|---|---|---|---|
| 1st | 192.168.1.0/26 | 192.168.1.1 – 192.168.1.62 | 62 |
| 2nd | 192.168.1.64/26 | 192.168.1.65 – 192.168.1.126 | 62 |
| 3rd | 192.168.1.128/26 | 192.168.1.129 – 192.168.1.190 | 62 |
| 4th | 192.168.1.192/26 | 192.168.1.193 – 192.168.1.254 | 62 |
Each /26 has 64 total IPs, 62 usable (first = network, last = broadcast).
Quick reference: bits borrowed → subnets
Frequently Asked Questions
How do you calculate the number of subnets?
Use the formula 2n where n is the number of host bits borrowed. Borrowing 2 bits from a /24 gives 4 subnets (/26 each). Borrowing 3 bits gives 8 subnets (/27 each). Each borrowed bit doubles the number of subnets while halving the hosts per subnet.
How many usable hosts are in a subnet?
Usable hosts = 2h − 2, where h is the number of host bits remaining. Subtract 2 for the network address (first IP) and broadcast address (last IP). A /26 has 6 host bits: 2⁶ − 2 = 62 usable hosts. A /28 has 4 host bits: 2⁴ − 2 = 14 usable hosts.
What is a subnet boundary?
A subnet boundary is the starting address of each subnet, which must fall on a power-of-two multiple of the subnet block size. For a /26 (block size 64), the valid boundaries in a /24 are .0, .64, .128, and .192. Subnets that do not start on a valid boundary are invalid.
Use the calculator to verify your subnetting work: