Subnet Calculator

Network is the first address and identifies the subnet itself; broadcast is the last. Neither is normally assigned to a host, so the usable range is everything in between. You can type any host address — it's masked down to its network automatically, so 10.0.5.77/22 gives you 10.0.4.0/22.

Netmask and CIDR prefix say the same thing: /24 is 255.255.255.0, meaning the first 24 bits are the network (highlighted in the mask bits). Each step down in prefix doubles the size — a /23 is two /24s. The wildcard is the mask inverted, which is what Cisco ACLs and OSPF network statements expect.

/31 and /32 are special: a /31 is a point-to-point link with both addresses usable (RFC 3021), and a /32 is a single host, used for loopbacks and host routes.

To check whether two devices are on the same subnet, enter one device's IP with the mask and see whether the other's IP falls inside the usable range. If it doesn't, traffic between them has to go through a router or gateway.