IP Subnet Calculator

Subnetting is the practice of dividing a larger IP network into smaller, independent sub-networks called subnets. Each subnet has its own address range and functions as a separate broadcast domain. This technique was introduced to solve the inefficiency of classful addressing, where organizations received far more IP addresses than they needed. By borrowing bits from the host portion of an IP address, administrators can create multiple smaller networks from a single address block, improving routing efficiency, reducing broadcast traffic, enhancing security through isolation, and making better use of limited IPv4 address space. Every IPv4 address is 32 bits long, divided into a network portion (identified by the subnet mask) and a host portion (the remaining bits). The subnet mask tells routers which part of the address identifies the network and which part identifies individual devices.

Classless Inter-Domain Routing (CIDR) replaced the rigid classful system (Class A, B, C) in 1993 with RFC 1519. Instead of fixed boundaries, CIDR uses a slash followed by a number to indicate how many bits form the network prefix. For example, /24 means 24 bits are the network portion and 8 bits are for hosts, yielding 2⁸ = 256 total addresses (254 usable). The key formula is: total addresses = 2^(32 − prefix), and usable hosts = total − 2 (subtracting network and broadcast addresses). A /16 gives 65,534 usable hosts, while /28 gives only 14. CIDR enables precise allocation — an organization needing 500 addresses can get a /23 (510 hosts) instead of wasting an entire Class B (/16 with 65,534). This flexibility dramatically reduced IPv4 address exhaustion and simplified routing tables through route aggregation (supernetting).