Obviously, we should also mention at this stage that CIDR requires routing protocols to carry 32-bit IP prefix masks as well as the 32-bit IP address. OSPF and RIP-2 are currently capable of carrying masks and BGP-4 is now being implemented by router vendors.
IP address prefixes provide hierarchical abstraction through a process known as summarization. By summarization, we mean that a pair of prefixes of length N can be summarized to a single prefix of length N-1 if the prefixes share the first N-1 bits. Example, the prefixes 1101 and 1100 can be summarized as 110. This can be applied repeatedly. The method of summarization is used to aggregate multiple routing entries into a single entry so that if for example, 8 sites are attached to the same NSFNET regional, and have summerizable addresses, then the regional can advertise its reachability to these 8 sites with just one IP address prefix.
Currently CIDR specifies 3 levels in the routing hierarchy where summarization can be done with room for further levels if needed. The three levels are.
Despite networks needing to be renumbered, for CIDR really to have an effect, packets will still be able to find their way. The reason for this is that Internet users and applications rarely refer to the IP address of a site but mostly the name. So renumbering does not impact the same way as renumbering a telephone number might.
Currently the BGP routing tables in the interdomain routing system indicate that a CIDR addressing hierarchy is capable of routing the current set of 10,000 networks with fewer than 200 routing entries. Changing host addresses of sites already in the internet would however be needed to achieve this.
Also it has been found in surveys that 50% of Class B networks have fewer than 50 hosts which could easily fit into a Class C network (254 hosts). Renumbering these addresses would reclaim a substantial amount of IP address space.
1993s figures showed the Internet as having 1.5 million hosts. By using CIDR it should be possible to map this to approximately 10% of the total address space. Thus the Internet should be able to grow to about 400 million systems, so that even if the internet doubles in size every year, it should be possible to route with IPv4 into the next century.