A subnet mask is a 32-bit number that divides an IPv4 address into a network portion and a host portion. In dotted-decimal form (for example 255.255.255.0) the bits set to 1 mark the network part and the bits set to 0 mark the host part.

CIDR (Classless Inter-Domain Routing, RFC 4632) writes a subnet as an IP address followed by a slash and the number of consecutive 1-bits in the netmask. For example, 192.168.1.0/24 represents a network of 256 addresses with 254 usable hosts.

A netmask marks network bits with 1s and host bits with 0s (for example 255.255.255.0). A wildcard mask is its bitwise inverse (0.0.0.255) and is used by Cisco ACLs to match address ranges.

RFC 1918 reserves three IPv4 ranges for private internal use that are not routed on the public internet: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.

A /24 contains 256 total addresses, but the network address (.0) and the broadcast address (.255) are reserved, leaving 254 usable host addresses.

RFC 3021 allows /31 prefixes for point-to-point links between two routers. Both addresses are usable, so a /31 provides exactly 2 hosts with no separate network or broadcast address.

Pre-CIDR classful addressing divided IPv4 into ranges by the high-order bits: Class A (0–127, default /8), Class B (128–191, /16), Class C (192–223, /24), Class D (224–239, multicast) and Class E (240–255, experimental). Modern routing is classless, but the labels are still used as a quick reference.

The network address is the first address in a subnet (host bits all 0) and identifies the subnet itself. The broadcast address is the last address (host bits all 1) and is used to send a packet to every host on the subnet.