CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing Internet Protocol packets in a more flexible way than with the original system of Internet Protocol (IP) address classes. CIDR addresses replace the subnet mask and state the number of bits that IP should use to determine the network portion of an IP address. A great deal of space can be saved in a routing table by using CIDR notation instead of the traditional subnet mask dotted decimal notation. Another enormous benefit of using CIDR is the flexibility in assigning network addresses. Instead of using the "classful" system of addressing, CIDR uses a "classless" system. There is no default mask with CIDR. A network address can be allocated with any number of bits representing the network portion of the address. CIDR addresses can be allocated based on the number of hosts; using this addressing system can suit a company better than using the classful system, which can waste addresses. CIDR Calc it is also intended to be a teaching tool.