IP Subnet Calculator

CIDR (Classless Inter-Domain Routing) notation writes an IP address followed by a slash and the prefix length — e.g., 192.168.1.0/24. The prefix length (0–32) is the number of leading bits that are fixed (the network portion). The remaining bits identify individual hosts within that network. /24 means the first 24 bits are the network, leaving 8 bits for 256 possible addresses (254 usable hosts).

RFC 1918 defines three private ranges: 10.0.0.0/8 (Class A, ~16 million addresses), 172.16.0.0/12 (Class B, ~1 million addresses), and 192.168.0.0/16 (Class C, 65536 addresses). These are not routable on the public internet and are used for internal networks behind NAT.

The network address (all host bits = 0) identifies the network itself and cannot be assigned to a host. The broadcast address (all host bits = 1) is used to send packets to all hosts on the subnet simultaneously. So a /24 network has 256 total addresses but only 254 usable hosts.

A wildcard mask is the inverse of the subnet mask — flip all 0s to 1s and vice versa. For /24 (mask 255.255.255.0), the wildcard is 0.0.0.255. Wildcards are used in Cisco ACLs and OSPF configurations to specify which bits to match (0 = must match, 1 = any value).