A supernet is a group of computer networks or subnetworks that are treated as a single entity. The concept was created in response to the shortcomings of the “classful” addressing system in which internet protocol (IP) addresses are distributed in pools of pre-defined size known as blocks. Supernetting allows organizations to customize the size of their networks and reduces the demand on network routing equipment by aggregating many separate routes.
With both supernets and classful addressing systems, IP addresses are split into a minimum of two portions: a network identifier that specifies a network, and a host identifier that specifies a computer or other device on that network. The overall length of an IP address is limited, so the size of one identifier limits the size of the other. Prior to the concept of a supernet, IP addresses were distributed in blocks, according to a “class” that determined how much of each address was dedicated to either type of identifier. In a “class A” address, the network identifier is quite short, leaving room for just 127 network blocks, while the host identifier’s relative length allows each of those 127 networks to have over 16 million hosts. The two other common classes are class B, which can support up to 65,534 hosts and 16,384 networks, and class C, which allows only 254 hosts, but just over two million networks.
The idea of a supernet was created in response to several problems with the classful addressing system. Many companies and organizations needed more than the 254 hosts available from a class C network block, but far fewer than the 65,534 addresses provided in a class B block. As a result, many medium sized organizations were assigned class B blocks but used only part of the 65,534 allotted addresses, leading to an inevitable shortage of class B addresses. In addition, the rapid growth of new websites and network destinations began placing a heavy burden on routing equipment which had to store more and more information in order to reach the increasing number of networks and hosts. In 1993, the Internet Engineering Task Force (IETF), officially endorsed the supernet concept to address these issues.
Supernetting, also known as classless interdomain routing (CIDR), eliminates the prior notion of classes. A supernet is essentially a group of smaller network blocks or subnetworks that is treated as a single large network. Network identifiers in a supernet can be of almost any length, allowing the size of the network to be customized according to an organization’s needs. Two class C blocks, for example, could be supernetted for a total of just over 500 addresses. This system also allows for route aggregation, which groups routing information for a variety of hosts or networks into a single “summarized” route.
The supernet concept has a few drawbacks, most notably the increased complexity in CIDR compared to the classful addressing system and the requirement for new routing protocols that supported CIDR. The ability to customize the length of a network identifier made it more difficult for system administrators to distinguish between network identifier and host identifier. To deal with this problem, a new style of writing IP addresses was introduced. In this style, which is called CIDR notation, or slash notation, a slash follows an IP address, which is followed by the number of bits used for the network ID. In the example 192.168.25.5/24, the first 24 bits of the address are the network identifier, while the remaining eight bits are the host identifier.