Subnet Mask and Prefix Converter

Two notations, one set of bits

A subnet mask and a prefix length say exactly the same thing in different alphabets. Both describe how many of an address’s thirty-two bits are fixed by the network.

The mask says it as thirty-two bits written in dotted decimal, with ones for the network part and zeros for the host part. 255.255.255.0 is twenty-four ones followed by eight zeros. The prefix just states the count: /24. Same bits, fewer characters.

Prefix notation won, and every modern tool uses it. Dotted-decimal masks persist because Cisco IOS interface configuration, Windows network settings and a great deal of existing documentation are written that way — which is why this conversion keeps coming up years after the notation was superseded.

Why only nine numbers are legal in an octet

The ones in a mask must be contiguous from the left. Within a single octet that permits nine values and no others:

Memorising that column is most of what “being good at subnetting by hand” amounts to. It also gives you a fast validity check by eye: any octet in a mask that is not one of those nine numbers is wrong, and any octet after a non-255 one must be 0.

The three masks that come up constantly

255.255.255.0 (/24) is the default on essentially every consumer router. 255.255.255.252 (/30) was the traditional point-to-point link mask — four addresses, two usable — now largely replaced by 255.255.255.254 (/31), which gives both addresses to hosts. And 255.255.0.0 (/16) is what Windows offers when it guesses, which is usually far larger than anyone wanted.

Doing it in your head

The trick is that only one octet is ever interesting. Everything to its left is 255, everything to its right is 0, and the prefix tells you which one and how far into it.

Divide the prefix by 8. The quotient is how many 255s; the remainder is how many bits are set in the next octet. For /26: 26 ÷ 8 is 3 remainder 2, so three 255s and then two bits — which the table above says is 192. 255.255.255.192.

Block size falls out of the same remainder: 256 minus the interesting octet. 256 − 192 = 64, so /26 blocks are 64 addresses and start at .0, .64, .128 and .192.

Related

To apply a mask to an actual address and get the network, broadcast and host range, the subnet calculator does the whole breakdown. To convert between blocks and address ranges in either direction, use the CIDR calculator.