Ipcidrsubnet Calculator

Every IPv4 address is a single 32-bit integer, and a CIDR prefix simply says how many of those bits identify the network. A /24 fixes the leading 24 bits, leaving 8 host bits, and the corresponding subnet mask 255.255.255.0 is that same split written as dotted decimal. All the derived values are bitwise operations on the integer: the network address is the address ANDed with the mask, the wildcard mask is the bitwise complement of the mask, and the broadcast address is the network ORed with the wildcard. Usable hosts are 2 to the power of the host bits minus 2, because the all-zeros host pattern names the subnet and the all-ones pattern is the directed broadcast. A /24 therefore carries 254 assignable addresses, a /26 carries 62, and a /28 carries 14. Two prefixes are exceptions: /31 provides exactly two addresses for point-to-point links with no broadcast, and /32 identifies a single host, commonly used for loopbacks and route entries. Sizing a subnet means counting every device that needs an address, adding 20 to 50 percent for growth, then choosing the smallest prefix whose host count exceeds that total. Fifty devices need a /26 rather than a /27. Oversizing enlarges broadcast domains, while undersizing forces renumbering. Internal addressing should come from the RFC 1918 private ranges 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, reached from the Internet through NAT. Note that the older Class A, B and C labels are historical descriptions of the first octet and no longer constrain how a prefix may be chosen, and that this arithmetic applies to IPv4 only.

The original 1981 specification of IP in RFC 791, edited by Jon Postel at the Information Sciences Institute, divided the address space into fixed classes: Class A used an 8-bit network part, Class B 16 bits and Class C 24 bits. The scheme was rigid, and a site needing 300 addresses had to take a Class B with 65,534. Subnetting was added in 1985 by RFC 950, written by Jeffrey Mogul and Jon Postel, which introduced the subnet mask so that an organization could split its assigned block internally without the outside world noticing. By the early 1990s two failures converged: Class B space was being consumed quickly, and backbone routing tables were growing faster than router memory. The answer was Classless Inter-Domain Routing, published in September 1993 in RFC 1518 and RFC 1519 by Vince Fuller, Tony Li, Jessica Yu and Kannan Varadhan. CIDR abolished the classes entirely, allowing a prefix of any length and, just as importantly, allowing providers to advertise many customer blocks as one aggregated route. RFC 4632 replaced RFC 1519 in 2006 as the current specification. Supporting pieces followed: RFC 1918 in February 1996 designated the private ranges still used on nearly every internal network, and RFC 3021 in December 2000 legitimized /31 prefixes on point-to-point links, halving the addresses wasted on router interconnects. CIDR extended IPv4 by decades, but the IANA free pool was finally exhausted on 3 February 2011.