Recall that classful addresses are decidedly wasteful in the way they allocate addresses –even an entire Class B address range is too large for most companies. To make matters worse, a Class C block is so small that many companies would require many Class C ranges as a viable alternative. On the public Internet, routing tables were becoming extremely large, which in turn was affecting routing performance. A new solution was required to deal with the public address space more efficiently, and came about via a method referred to as Classless Inter-Domain Routing (CIDR).
CIDR is sometimes referred to as supernetting. While subnetting involves taking an address space and breaking it up into a number of smaller networks, supernetting is first and foremost a network aggregation scheme. For example, by supernetting four network addresses together, you can actually make them appear to be one network, as opposed to four. Again, this is all accomplished by playing with the subnet mask values.
CIDR has its own notation for dealing with subnet masks, and you may already be familiar with it. Instead of taking the time to say network 131.107.0.0 with a subnet mask of 255.255.0.0, CIDR notation truncates the subnet mask to what is known as “slash” notation. In this example, the network would be identified as 131.107.0.0/16. The “/16” value refers to the fact that the first 16 bits in the subnet mask are all set to values of binary 1.
Figure: Determining CIDR notation based on a subnet mask.
This really is a more efficient way of referring to a network. For example, if we had a network address of 192.168.0.0 with a mask of 255.255.255.128, in CIDR notation it becomes 192.168.0.0/25. Again, the /25 means that the first 25 bits of the subnet mask are set to binary 1.
But what is supernetting really? Well, imagine a routing table on a large Internet backbone router. This router needs to know how to get to all public networks. If a certain company has been given many different network IDs (for example, an ISP might have 8 Class B network IDs), a minimum of 8 routing table entries would be required, just to be sure that these 8 networks could be reached. As routing tables get larger and larger, routing performance is definitely impacted. Supernetting provides a way to “collapse” or “summarize” those 8 entries into a single entry, by altering the subnet mask value.
Tip: Supernetting allows contiguous network addresses to be summarized into a single routing table entry through the use of a custom subnet mask.
The first requirement to supernet addresses together is that network addresses must be contiguous. For example, you could supernet together networks 131.107.0.0 and 131.108.0.0, but not networks 131.107.0.0 and 145.36.0.0.
