Subnet calculator (CIDR, IPv4 and IPv6)

An IP address is a number: 32 bits for IPv4, 128 for IPv6. CIDR notation like 192.168.1.0/24 says how many of those bits, counted from the left, name the network (24 here). The bits that are left over number the devices inside it, so a /24 has 8 host bits and 2⁸ = 256 addresses. The coloured bits above show the split, and moving the slider moves the line between them.

In IPv4 the first address of a block is the network address and the last is the broadcast address, so a /24 leaves 254 addresses for devices. Two exceptions: a /31 is a point-to-point link where both addresses are usable, and a /32 is a single address. IPv6 has no broadcast address, so every address in the block counts.

The subnet mask is the same split written as an address (/24 is 255.255.255.0), and the wildcard mask is its inverse (0.0.0.255), as used in Cisco access lists and OSPF. Splitting a block lists the smaller subnets it divides into, in order.

It works on one block at a time. It doesn't yet turn an address range like 10.0.0.5 to 10.0.0.20 into CIDR blocks, or merge several blocks into one.

A split shows the first 256 subnets; the count above the list is always the full number. Address types (private, documentation, multicast and so on) come from the IANA special-purpose registries; anything else is shown as public.

The block you enter is kept after the # in the page address, so you can bookmark or share it. That part of an address is never sent to a server.