Subnet Splitter (VLSM)

The subnet splitter divides one IPv4 CIDR block (e.g. 192.168.0.0/24) into smaller, equal-sized subnets and shows each child subnet's network address, first/last usable host, broadcast address and usable host count at a glance. Before you start a VLSM (Variable Length Subnet Mask) design, it answers "if I split a /24 into /26s, how many do I get and where does each range end?" in one step.

Enter a base CIDR and a new prefix length (a number larger than the base prefix). The number of child subnets is 2^{(new prefix − base prefix)}. To inspect the host range of a single block, use the CIDR calculator; to fold a start/end IP pair back into CIDR, use the IP range to CIDR converter.

How subnet splitting works

When you choose a new prefix longer than the base, the number of subnets grows as a power of two for each added bit. Splitting a /24 into /26 yields 2^(26−24) = 4 subnets, and each holds 2^(32−26) = 64 addresses (62 usable hosts).

• Network address: the first address of the subnet (not usable as a host).
• First usable host: network address + 1.
• Last usable host: broadcast − 1.
• Broadcast: the last address of the subnet.
• Usable hosts: 2^{(32−prefix)} − 2.

The /31 and /32 special cases

The usual formula subtracts two addresses (network and broadcast), but a /31 used for point-to-point links treats both addresses as usable per RFC 3021, giving 2 usable hosts. A /32 is a single host (loopback, load-balancer VIP, etc.) and is shown as 1. This tool applies these exceptions automatically.

Even if the base network is given on an unaligned address (e.g. 192.168.0.5/24), the tool first normalizes it to the true network address (192.168.0.0) before splitting, so results always land on clean boundaries.

Quick reference: splitting a /24 into smaller prefixes

Here is the most common case — carving a single /24 into smaller prefixes — laid out in a table. Each extra bit in the new prefix doubles the number of subnets and roughly halves the usable hosts per subnet. The usable host count is 2^{(32−prefix)} − 2.

Worked example: splitting 192.168.1.0/24 into /26

Take the base block 192.168.1.0/24 (256 addresses) and split it into /26. That yields 2^(26−24) = 4 subnets, each holding 64 addresses (62 usable hosts). Notice how the last octet steps by 64: 0 → 64 → 128 → 192.

Together the four subnets cover the original 256 addresses (0–255) with no gaps: the address right after the first subnet's broadcast (.63) is the network address of the second subnet (.64), and so on. Every block shares the mask 255.255.255.192. To cross-check a single block's mask and wildcard, run it through the CIDR calculator.