IP addressing is the foundational addressing system of the internet, and subnetting is the practice of dividing a larger network into smaller, more manageable segments. Every device on a network receives an IP address, which consists of two parts: the network portion that identifies which network the device belongs to, and the host portion that identifies the specific device within that network. The subnet mask determines where this boundary falls, and understanding this division is essential for network design, routing, and security.

Historically, IP addresses were divided into rigid classes. Class A networks (first octet 1-126) used an 8-bit network prefix and could contain over 16 million hosts. Class B networks (128-191) used 16 bits for the network and supported 65,534 hosts. Class C networks (192-223) used 24 bits and allowed only 254 hosts. This classful system was enormously wasteful because organizations received far more addresses than they needed. An organization needing 300 addresses would receive an entire Class B allocation of 65,534 addresses, leaving tens of thousands unused.

Classless Inter-Domain Routing (CIDR), introduced in 1993 through RFC 1518 and RFC 1519, replaced this rigid system with variable-length subnet masks. CIDR notation appends a prefix length to the IP address (e.g., 192.168.1.0/24), indicating exactly how many bits form the network portion. This allows networks to be sized precisely to requirements. A /25 gives 126 usable hosts, a /26 gives 62, and a /27 gives 30. The mathematics of subnetting involves binary operations: the subnet mask is a contiguous sequence of 1-bits followed by 0-bits, and the network address is derived by performing a bitwise AND between the IP address and the subnet mask. The broadcast address is found by setting all host bits to 1.

IPv6 dramatically expands the address space from 32 bits to 128 bits, providing approximately 340 undecillion unique addresses. While IPv6 eliminates the address exhaustion problem that drove much of IPv4 subnetting complexity, subnetting remains important for network organization and security segmentation. IPv6 subnets typically use a /64 prefix for local networks, with the remaining 64 bits available for host addressing through Stateless Address Autoconfiguration (SLAAC). The wildcard mask, used primarily in access control lists on Cisco routers, is the bitwise inverse of the subnet mask and defines which bits in an address should be matched (0) versus ignored (1) when evaluating network rules.