CIDR Subnet Calculator network

What this tool does

CIDR notation expresses a network range by appending a slash and a number to an IP address. In 192.168.1.0/24 the 24 means the first 24 bits identify the network and the remaining 8 bits address hosts, giving 256 addresses in the block.

Only 254 of those can be assigned to equipment. The first address identifies the network itself and the last is reserved as the broadcast address. Larger prefixes mean smaller blocks: /25 holds 128 addresses, /26 holds 64, and /30 holds 4 with only 2 usable.

Enter an address and prefix and this tool reports the network address, subnet mask, wildcard mask, broadcast address, first and last host, total addresses, and usable host count. Supplying an address from the middle of a range still resolves the network it belongs to.

When to use it

Use it to confirm the range for a firewall or security group rule, to divide a VPC into subnets, to determine which block an address belongs to, to plan a DHCP pool, or to check whether routing table entries overlap.

Input and output examples

Notes and limitations

This tool covers IPv4 only; IPv6 uses a different addressing scheme and needs separate calculation. The /31 and /32 prefixes are exceptions to the usual rules: /32 identifies exactly one host, and /31 is defined for router-to-router links where both addresses are usable and there is no broadcast address. Cloud providers also reserve addresses inside each subnet for their own use, with AWS taking five per subnet, so real availability is lower than the arithmetic suggests. Check the provider documentation when planning ranges.

Frequently asked questions