IP Range Calculator (CIDR ↔ Range)

Firewall rules, cloud security groups, VPN routes and allow-lists are written either as CIDR blocks (10.0.0.0/22) or as start–end ranges. This calculator converts in both directions for IPv4: give it a CIDR block and it lists the first and last address, usable hosts, subnet and wildcard masks; give it a range and it finds the smallest set of CIDR blocks that covers exactly those addresses — nothing more, nothing less.

How to use this tool

1. Type a CIDR block (192.168.1.0/24), an address with a mask (192.168.1.0 255.255.255.0) or a range (10.0.0.5 - 10.0.0.90).
2. For a range in one subnet you can shorten the end, as in 10.0.0.5-90.
3. Read the range, address counts and masks; for a range, copy the CIDR list.
4. Read address-type notes as a partial special-use lookup; actual allocation and routability are not checked.

Formula

Addresses in a /p block = 2^(32 − p). Network = IP − (IP mod 2^(32 − p)); last address = network + 2^(32 − p) − 1.

For ordinary unicast subnets, conventional host capacity = total − 2, because the first address names the network and the last is the broadcast address. /31 links use both addresses (RFC 3021) and /32 is a single host. To split a range, the calculator repeatedly takes the largest aligned power-of-two block that starts at the current address and does not pass the end.

Example

The range 192.168.1.10 – 192.168.1.100 holds 91 addresses. No single CIDR block fits it exactly, so it splits into 7 blocks: 192.168.1.10/31, .12/30, .16/28, .32/27, .64/27, .96/30 and .100/32 (2 + 4 + 16 + 32 + 32 + 4 + 1 = 91). Entering 172.16.5.77/20 instead shows the block 172.16.0.0 – 172.16.15.255 with 4,094 usable hosts.