Subnet Calculator

A CIDR prefix length states directly how many of the 32 address bits are frozen as the network identifier, which makes subnet sizing a matter of counting rather than of memorising mask octets. The remaining 32 minus p bits form the host field, so the block spans 2 to that power addresses and offers two fewer assignable ones. The practical table runs from /24 with 254 hosts, through /25 with 126, /26 with 62, /27 with 30 and /28 with 14, down to /30 with exactly two, the classic serial or point-to-point link. RFC 3021 permits /31 on point-to-point links where no broadcast is needed, and /32 denotes a single host route. Because blocks must align to their own size, a prefix always starts at a multiple of its block length: 10.0.5.67/26 is a host inside 10.0.5.64/26, not the start of a network. Sizing decisions are driven by two competing pressures. Too small a prefix leaves no headroom once the default gateway, DHCP relay, first-hop redundancy virtual addresses, printers, and access points are counted, and renumbering later is disruptive. Too large a prefix inflates the layer-2 broadcast domain, which is why most enterprise designs cap general-purpose VLANs at /24 or smaller: ARP and broadcast traffic scale with host count, and a single spanning-tree or flooding fault affects everyone inside the domain. A third consideration is aggregation. Because contiguous, properly aligned blocks summarize cleanly, four adjacent /26 assignments can be advertised upstream as one /24, keeping routing tables small. Non-contiguous masks are invalid, and unaligned allocations destroy that summarization.

Classless Inter-Domain Routing was an emergency measure. By the early 1990s the class system was failing on two fronts at once: an organisation needing 500 addresses had to be given a Class B block of 65,536, wasting most of it, while the global routing table was growing faster than router memory could absorb. The IETF convened the Routing and Addressing group to address both, and the result was CIDR, specified by Vince Fuller, Tony Li, Jessica Yu, and Kannan Varadhan in RFC 1338 in June 1992, revised as RFC 1519 in September 1993, and restated in the current RFC 4632 of August 2006. CIDR could only work once routing protocols carried a prefix length alongside each network number, since the class of an address no longer implied its mask. BGP-4, defined in RFC 1654 in 1994, was built for exactly this and remains the protocol that carries the internet routing table. Inside organisations, variable-length subnet masking became usable when OSPF, first published as RFC 1131 in 1989 and revised as OSPFv2 in RFC 2328 in 1998, and RIP version 2, published as RFC 1723 in 1994, began advertising masks with each route. The remaining rough edge was addressed in RFC 3021 in December 2000, which allowed /31 prefixes on point-to-point links and recovered the two addresses that /30 assignments had been wasting on every router-to-router hop.