Subnet calculator

How to use this tool

1. Enter an address and prefix length, or append a CIDR suffix such as /24 or /64.
2. Inspect the network start, final address and total block size.
3. Check reservation assumptions before copying the host counts or address rows.

An IPv4 /31 point-to-point block

Input and settings

192.0.2.11/31

Result

Network 192.0.2.10/31; two addresses; both count as hosts

The usual two-address network/broadcast subtraction is not applied to a /31 link.

The entered address need not be the block start

The prefix identifies network bits and the remainder identifies host bits. Applying the mask finds the network start; filling the host bits with ones finds the block’s last address. The bar depicts the split between those bit ranges.

A prefix attached to the address takes precedence over the separate prefix field. IPv4 permits 0–32 and IPv6 permits 0–128. Big-integer arithmetic preserves even the address count of an exceptionally large block such as /0.

Mathematical counts and deployable addresses

Ordinary IPv4 blocks subtract network and broadcast addresses. Under point-to-point /31 semantics both addresses are counted, and /32 denotes one address. Extra reservations made by a cloud platform must be subtracted separately from this mathematical count.

For IPv6 the output shows the entire range and count without inventing a broadcast address. It does not query current availability, ownership, firewall configuration or routing on the internet.

Common questions