IPv4 Subnet Calculator

An IPv4 address is 32 bits. The CIDR prefix says how many of those bits name the network; the rest number the hosts inside it. A /24 keeps 24 network bits and leaves 8 for hosts, so the network holds 2⁸ = 256 addresses.

The calculator clears the host bits to find the network address and sets them all to 1 to find the broadcast address. Every address in between is a usable host, which is why usable hosts are 2^{host bits} - 2. The wildcard is the mask with every bit flipped, the form Cisco ACLs and OSPF use.

Two sizes follow different rules: a /31 has two usable addresses for a point-to-point link (RFC 3021), and a /32 is a single host.

Take 192.168.1.10/26. A /26 leaves 6 host bits, so each block is 64 addresses and blocks start at .0, .64, .128 and .192. The address .10 falls in the first block:

Nothing. They are two ways to write the same mask: 24 one-bits followed by 8 zero-bits.

Find the smallest power of two that is at least hosts + 2. For 50 hosts that is 64, a /26. If you need several subnets of different sizes from one block, use the VLSM Planner.

Some systems read a leading zero as octal, so 010 can mean 8 instead of 10. To avoid a silent wrong answer, octets with leading zeros are refused.

Yes. Enter something like 10.20.30.40/20 in the address box and the calculator uses the pasted /20 instead of the prefix field, which keeps its old value. A note under the fields shows which prefix is in use.