Subnet mask - CIDR - Subnet Calcuator

CIDR

Class A addresses are long gone, and the world is quickly running out of Class B addresses. Many Class C addresses are still available, but the small address space of a Class C network (254 hosts maximum) is a severe limitation in the high-volume game of Internet service providers (ISPs).

It is possible to assign a range of Class C networks to a network owner who needs more than 254 addresses. However, treating multiple Class C networks as separate entities when they are all going to the same place only clutters up routing tables unnecessarily.

Classless Internet Domain Routing (CIDR) is a technique that allows a block of network IDs to be treated as a single entity in routing tables. CIDR groups a range of network IDs into a single address entry using what is called a supernet mask.

You can think of a supernet mask as something like the opposite of a subnet mask. Instead of designating additional bits for identifying the network, the supernet mask in effect takes bits away from the network ID. The addresses in the range are therefore identified by the network address bits that the networks in the range hold in common.

Subnet Calcuator

Subnets calculator will calculate numbers of subnet for ip address range and make CIDR summarisation for given ip address. CIDR Ranges format (CIDR summarisation) is very useful if you need to block some ip ranges on your server using .htaccess