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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 Subnet Addressing ("Subnetting") Concepts

IP Subnet Addressing ("Subnetting") Concepts 1 2 3 IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing

IP addressing was originally designed around the assumption of a strict two-level hierarchy for internetworks. The first level was the network, and the second level the host. Each organization was usually represented by a single network identifier that indicated a Class A, B or C block dedicated to them. Within that network they had to put all of the devices they wanted to connect to the public IP network.

It did not take long after the “classful” scheme was developed for serious inadequacies in it to be noticed, especially by larger organizations. The reason is that while dividing a large internetwork into networks that contain hosts is conceptually simple, it doesn't always match well the structure of each of the networks that comprises the internet. A big company with thousands of computers doesn't structure them as one big whomping physical network. Trying to assign and administer IP addresses to an organization's entire network without any form of internal logical structure is very difficult.

Unfortunately, under the original “classful” addressing scheme, there was no good solution to this problem. The most commonly-chosen alternative at the time was to trade a single large block of addresses such as a Class B for a bunch of Class Cs. However, this caused additional problems:

• It contributed to the explosion in size of IP routing tables.
  
  
• Every time more address space was needed, the administrator would have to apply for a new block of addresses.
  
  
• Any changes to the internal structure of a company's network would potentially affect devices and sites outside the organization.
  
  
• Keeping track of all those different Class C networks would be a bit of a headache in its own right.

IP Subnet Addressing ("Subnetting") Concepts 1 2 3 IP Subnetting: "Three-Level" Hierarchical IP Subnet Addressing