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 "Classful" (Conventional) Addressing

IP Multicast Addressing 1 2 3 IP Subnet Addressing ("Subnetting") Concepts

The “classful” addressing system was the first major attempt to define a method for universal addressing of a large IP internetwork. The system had some advantages, as I mentioned in the overview of the “classful” scheme, and given that it was developed decades ago for a network that was limited in size, it did the job remarkably well for a long time.

Nobody ever expected the Internet to mushroom to anything close to its current size. As it grew, problems become apparent with the “classful” IP addressing mechanism—slowly at first, but then more rapidly as growth became more rapid. I've hinted at some of theses problems in my explanation of how this type of addressing works, but to help frame the discussion of newer addressing types, I think it is useful to look at this in more detail.

There are three main problems with “classful” addressing, which are somewhat related to each other (making them a bit harder to explain). Let's start with a quick summary of what these issues are:

1. Lack of Internal Address Flexibility: Big organizations are assigned large, “monolithic” blocks of addresses that don't match well the structure of their underlying internal networks.
   
   
2. Inefficient Use of Address Space: The existence of only three block sizes (classes A, B and C) leads to waste of limited IP address space.
   
   
3. Proliferation of Router Table Entries: As the Internet grows, more and more entries are required for routers to handle the routing of IP datagrams, which causes performance problems for routers. Attempting to reduce inefficient address space allocation leads to even more router table entries.

Issue #1 results primarily from the fact that in the “classful” system, big companies get assigned a rather large (Class B) or truly enormous (Class A) block of addresses, all of which is considered by the Internet routers a single “network” with one “network ID”. Now, imagine that you are running a medium-to-large-sized company with 5,000 computers, and you are assigned a Class B address for your network. Do you really have 5,000 computers all hooked into a single network? I sure as heck hope you don't! Yet you would be forced to try to fit all of these into a single IP “network” in the original “classful” method. There was no way to create an internal hierarchy of addresses.

IP Multicast Addressing 1 2 3 IP Subnet Addressing ("Subnetting") Concepts