Subnet Calculator

An IPv4 address is 32 bits. The number after the slash says how many of those bits name the network; the rest number the hosts inside it. A /24 leaves 8 host bits, so it holds 2⁸ = 256 addresses. Every step down halves the block: a /25 holds 128, a /26 holds 64.

In a normal subnet the first address is the network itself and the last is the broadcast address, so neither can be given to a server. That is why a /24 has 254 usable hosts, not 256. Two exceptions: a /31 is a point-to-point link where both addresses are usable (RFC 3021), and a /32 is a single host.

IPv6 has no broadcast address, and subnets are far larger. The usual size for one network is a /64; providers commonly hand customers a /56 or /48.

The netmask is the prefix written as four numbers: /27 is 255.255.255.224. The wildcard mask is its inverse (0.0.0.31) and is what Cisco ACLs and OSPF statements expect. Both are shown so you can paste whichever your router or firewall wants.

Hosting networks are usually carved from a /24 or larger: a /29 or /28 per dedicated server, single /32s routed to VMs, and a /30 or /31 for each uplink. Routing /32s to VMs, rather than giving each customer a slice of a shared subnet, avoids losing a network and broadcast address per customer and stops one VM claiming its neighbour's address.