IPv4 - Supernetting (CIDR)

• supernetting also called Classless Addressing or Classless Inter-Domain Routing (CIDR)
• main goal of CIDR: to balance the desire to minimize the number of routes that a router needs to know against the need to hand out addresses efficiently
• How CIDR works:
  • suppose one needs 8 class C address
  • instead of assigning 8 addresses at random, we can hand out a block of contiguous class C addresses such as 128.112.128/24 through 128.112.135/24
  • this results in a common 21 bit network prefix: 128.112.128/21
  • and thus, we get both the high address efficiency of handing out addresses in chunks smaller than a class B network, and a single network prefix that can be used in forwarding tables
• CIDR requires a new type of notation to represent network numbers, or prefixes as they are known, because the prefixes can be of any length. The convention is to place a /X after the prefix, where X is the prefix length in bits. So, for the example above, the 21-bit prefix for all the networks 128.112.128/24 through 128.112.135/24 is represented as 28.112.128/21.
• today, with CIDR being the norm, it is more common to hear people talk about “slash 24” prefixes than class C networks
• representing a network address in this way is similar to the ⟨subnet-mask, value⟩ approach used in subnetting, as long as masks consist of contiguous bits starting from the most significant bit (which in practice is almost always the case)

Now imagine using CIDR in a Internet Service Provider (ISP) network, whose primary job is to provide Internet connectivity to a large number of corporations and campuses (customers)