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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting" 1 2 3 IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation

Subnet addressing was an important development in the evolution of IP addressing, because it solved some important issues with the conventional, two-level class-based addressing scheme. Subnetting's contribution to ease in IP addressing was allowing each network to have its own two-level hierarchy, giving the administrator of each network the equivalent of an “internet within the Internet”.

When we looked at the advantages of subnetting, we saw that one was that subnetting was local within each organization, and “invisible” to other organizations. This is an advantage in that it lets each organization tailor its network without other groups having to worry about the details of how this is done. Unfortunately, this “invisibility” also represents a key disadvantage of subnetted “classful” addressing: it cannot correct the fundamental inefficiencies associated with that type of addressing, because organizations are still assigned address blocks based on classes.

A key weakness of regular subnetting is low “granularity”. A Class B address block contains a very large number of addresses (65,534) but a Class C block has only a relatively small number (254). There are many thousands of “medium-sized” organizations who need more than 254 IP addresses, but a small percentage of these need 65,534 or anything even close to it. (The lack of a good match to a medium-sized organization with 5,000 hosts is illustrated in Figure 64.) When setting up their networks, these companies and groups would tend to request Class B address blocks and not Class C blocks because they need more than 254, without considering how many of the 65,000-odd addresses they really would use.

Now, due to how the classes of the older system were designed, there are over 2 million Class C address blocks, but only 16,384 Class Bs. While 16,384 seems like a lot at first glance, there are millions of organizations and corporations around the world. Class B allocations were being consumed at a rapid pace, while the smaller Class C networks were relatively unused.

The folks handing out Internet addresses needed a way to better utilize the address space so it would not run out before the transition to IP version 6. Subnetting didn't help a great deal with this problem. Why? Because it only works within the “classful” address blocks. If an organization needing 2,000 IP addresses requests a Class B block, they could use subnetting to more efficiently manage their block. However, subnetting could do nothing about the fact that this organization would never use over 62,000 of the addresses in its block—about 97% of their allocated address space.

The only solution to this would be to convince—or at worst case, force—companies to use many smaller Class C blocks instead of “wasting” the bulk of a Class B assignment. Many organizations resisted this due to the complexity involved, and this caused the other main problem that subnetting didn't correct: the growth of Internet routing tables. Replacing one Class B network with 10 Class Cs means ten times as many entries for routers to keep track of.

IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting" 1 2 3 IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation