IPv4 subnet calculator

The formula, with a worked example

• Mask = the prefix length in ones, then zeros: /24 is 24 ones and 8 zeros, which is 255.255.255.0.
• Network address = IP address AND mask: 192.168.1.10 AND 255.255.255.0 = 192.168.1.0.
• Wildcard = NOT mask: 255.255.255.255 minus 255.255.255.0 = 0.0.0.255.
• Broadcast address = network OR wildcard: 192.168.1.0 OR 0.0.0.255 = 192.168.1.255.
• Total addresses = 2^(32 − prefix): 2^8 = 256.
• Usable hosts = total − 2 (the network and broadcast addresses): 256 − 2 = 254, from 192.168.1.1 to 192.168.1.254.
• Exceptions: a /31 has 2 usable addresses (RFC 3021) and a /32 has 1.

What the results mean

An IPv4 address is 32 bits, written as four numbers from 0 to 255. The prefix length says how many of those bits, counted from the left, name the network. The rest name a host inside it. In 192.168.1.10/24 the first 24 bits (192.168.1) are the network part and the last 8 bits (.10) are the host part.

The network address is the first address of the subnet, with every host bit set to 0. The broadcast address is the last, with every host bit set to 1; a packet sent there goes to every host on the subnet. Everything in between is the usable host range, the addresses that can be given to devices. The subnet mask and the wildcard mask are two ways of writing the same split: the mask has ones over the network bits, and the wildcard has ones over the host bits.

The address type line says whether the address sits in one of the special-purpose ranges listed by IANA, such as the private ranges from RFC 1918 or the documentation ranges used in examples. Anything outside them is shown as public.

Working it out by hand, step by step

Take 10.20.30.40/22. A /22 mask is 22 ones and 10 zeros: 11111111.11111111.11111100.00000000, which is 255.255.252.0. Only the third number is split, so only the third number needs real work. 30 in binary is 00011110. AND it with the mask byte 11111100 and you get 00011100, which is 28. The network address is 10.20.28.0.

The block size in the third number is 256 − 252 = 4, so this subnet covers 28, 29, 30 and 31 in that position. The broadcast address is therefore 10.20.31.255, the usable range runs from 10.20.28.1 to 10.20.31.254, and the subnet holds 2^10 = 1,024 addresses, 1,022 of them usable.

That "256 minus the mask number" shortcut works for any prefix. In the octet where the mask stops being 255, subtract the mask value from 256 to get the block size, then find the multiple of the block size at or below the address's value in that octet. That multiple is where the network starts.

Prefix length or subnet mask: either works

The second field takes a prefix length (24 or /24) or a dotted mask (255.255.255.0). They mean the same thing. A valid mask is always a run of ones followed by a run of zeros, so the only numbers that can appear in a mask are 0, 128, 192, 224, 240, 248, 252, 254 and 255, and once one octet is below 255 every octet after it is 0. A mask like 255.0.255.0 has gaps, and the calculator says so instead of guessing.

You can also type the whole thing in CIDR notation in the first field, for example 172.16.5.130/26. The prefix after the slash then wins over whatever is in the second field.

Edge cases: /31, /32, /0 and the special ranges

A /31 holds two addresses. Under the old rule both would be lost to the network and broadcast addresses, leaving none for hosts. RFC 3021 lets both addresses be used on a point-to-point link, so the calculator counts 2 usable addresses and shows the range as the whole block. For 203.0.113.7/31 that is 203.0.113.6 to 203.0.113.7.

A /32 is a single address on its own, sometimes called a host route. The network, broadcast, first and last address are all the same, and it counts as 1 usable address. At the other end, a /0 covers every IPv4 address, all 4,294,967,296 of them, with a mask of 0.0.0.0.

The address type is a label from the IANA registry, nothing more. 100.64.0.0/10, for example, is shared address space set aside for carrier-grade NAT, and 169.254.0.0/16 is link-local, the range a device picks from when it has no other address.