IP Subnet Calculator – calculate subnet masks online

In CIDR notation, the prefix length follows the slash: with /24, 24 bits belong to the network and 8 bits to the hosts, corresponding to subnet mask 255.255.255.0. For 192.168.1.0/24, the calculator shows network address 192.168.1.0, broadcast address 192.168.1.255, wildcard mask 0.0.0.255, and usable host range 192.168.1.1 to 192.168.1.254—that is 254 usable hosts out of 256 addresses.

The prefix length determines how many bits of the subnet mask are set to 1. For example, /26 gives 255.255.255.192. The network address is the result of a bitwise AND between the IP address and subnet mask; it is the first address in the subnet. To get the broadcast address, set all host bits to 1; it is the last address. The calculator does both as you type and also shows the subnet mask in binary. You can find the full calculation steps further up this page.

The first address identifies the network itself (network address), while the last is reserved for broadcast messages to all devices in the subnet (broadcast address). That is why a subnet normally has two fewer usable host addresses than total addresses. For /31 (point-to-point links under RFC 3021) and /32 (a single host address), these addresses are not reserved, so the calculator counts every address as usable.

The wildcard mask is the inverse of the subnet mask (e.g., 0.0.0.255 instead of 255.255.255.0). It is primarily used in Cisco ACLs (Access Control Lists) and OSPF configurations, where address ranges are defined using wildcard masks rather than subnet masks.

Without “/prefix length”, subnet mode assumes /24 and marks the prefix length as “assumed”. The input field does not accept a subnet mask in dotted-decimal notation; it expects CIDR notation with a prefix length from 0 to 32. Convert the mask first, for example 255.255.255.0 to /24. The subnet mask table on this page can help. In “CIDR ↔ IP Range” mode, however, an address without a prefix is treated as a single address.

In “CIDR ↔ IP Range” mode, the calculator shows the first and last address of a CIDR block and the number of addresses: 192.168.0.0/16 runs from 192.168.0.0 to 192.168.255.255 and contains 65,536 addresses. To convert the other way, enter a start and end address. A range fits into a single block only if it starts and ends on the right boundaries. Otherwise, the converter returns the smallest possible list of CIDR blocks: 10.0.0.5 to 10.0.0.20 produces five blocks that you can copy together.

Partly. “CIDR ↔ IP Range” mode works with 128-bit addresses and shows the exact address count, even for a /64 with 18,446,744,073,709,551,616 addresses. The start and end addresses must both be IPv6; mixing IPv6 and IPv4 is not supported. Subnet mode, with its subnet mask, broadcast address, and host range, is IPv4-only.

Yes. In “CIDR ↔ IP Range” mode, you can download all addresses as TXT (one address per line) or as CSV with a header row. The file is generated directly in your browser, just like the calculation itself: nothing is sent to a server when you calculate. Exports are limited to 1,048,576 addresses, the number in an IPv4 /12 block. An IPv6 /64 therefore cannot be listed; use its start and end addresses instead.