CIDR prefix lengths

Every IPv4 prefix length, its mask and how many hosts it holds. Use the subnet calculator to work out a specific network's range.

Reading a prefix length

An IPv4 address is 32 bits. The prefix says how many of the leading bits identify the network; whatever is left identifies a host inside it. The counter-intuitive part is that a smaller number means a larger network, because fewer network bits leaves more host bits. A /16 is 256 times the size of a /24.

Every step of one bit doubles or halves the network, which is why subnet sizes are always powers of two. There is no way to allocate a network of exactly 300 addresses: you take a /23, which holds 512, and leave the remainder unused.

The two lost addresses

In any subnet of /30 or larger, two addresses cannot be assigned to a machine. The lowest — all host bits zero — is the network address and names the subnet itself. The highest — all host bits one — is the broadcast address. A /24 therefore holds 256 addresses but only 254 hosts.

The waste is negligible at /16 and severe at /30, where half the block is lost. That is exactly why /31 exists: RFC 3021 removes the broadcast address on point-to-point links, making both addresses usable and halving the waste on router-to-router connections.

The sizes you will actually meet

Working out a mask without a calculator

Only nine values can appear in an octet of a subnet mask, because each is a run of leading ones:

Divide the prefix by 8 to get the number of complete 255 octets, and the remainder picks the next octet from this table. A /26 is three complete octets and a remainder of 2, giving 255.255.255.192. If a mask contains any other number, it is wrong.