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  IP Network Address Translation (NAT) Protocol

IP Network Address Translation (NAT) Protocol 1 2 3 4 5 IP NAT Address Terminology

Indirect Internet Connectivity

One solution to the problems of IP address space and security was to set up a system where a company's network was not connected directly to the Internet, but rather indirectly. Setting up a network this way is possible due to several important characteristics of how most organizations use the Internet:

• Most Hosts Are Client Devices: The Internet is client/server based, and the majority of hosts are clients. Client devices generally don't need to be made publicly accessible. For example, when using your local PC to access the World Wide Web, you issue requests to servers and they respond back, but servers don't have any reason to try to initiate contact with you. Most correspondence is begun by clients and not servers, by definition.
  
  
• Few Hosts Access The Internet Simultaneously: When you have a large number of hosts on the same network connected to the Internet, at any given time usually only a small number of them are trying to access the 'net. It isn't necessary to assume they will all need to access servers at once. Even while you actively browse the Web, you pause for a number of seconds to read information from time to time; you are only accessing the Web server for the time it takes to perform a transaction.
  
  
• Internet Communications Are Routed: Communications between an organization's network and the Internet go through a router, which acts as a “control point” for traffic flows.

The best way to explain why these attributes matter is to draw an analogy to how telephones are used an organization, because many of the same attributes apply there. Most of the telephones in a typical organization are used to let employees make phone calls out. Usually there is no need to have any way to call employees directly; instead one system or person can handle all incoming calls. Only a few employees are ever making a call to the “outside world” at any given time. And all calls are routed through a central point that manages the telephone system.

For these reasons, to save money, organizations don't run separate public telephone lines to every employee's desk. Instead, it sets up a telephone system where each employee gets an extension, which is basically a local telephone number valid only within the organization. A small number of outside lines is made available in a pool for employees to share, and the telephone system matches the inside extensions to the outside lines as needed. A voice mail system and/or human receptionist handle routing of calls in to the organization.

(Yes, some companies have a direct mapping between extension numbers and real telephone numbers. Don't make trouble. J)

IP Network Address Translation (NAT) Protocol 1 2 3 4 5 IP NAT Address Terminology