CertiGuide to A+ (A+ 4 Real)  9  Chapter 14: Networks       9  What is a Network?            9  Protocol Standards

Internet Protocol (IP) 1 Automatic Private IP Addressing (APIPA)

A process called Network Address Translation (NAT) is one method of solving this problem. NAT runs on the gateway (a device such as a router) between the internal network and the Internet, receives all requests from the internal private addresses and remembers them in an internal “NAT table”. It then repackages all the requests to make them appear to be using the public IP, and sends them out to the Internet. When hosts on the Internet respond to these requests, they see the organizations public IP address as the origin. When replies are returned to the gateway running NAT, it looks at the NAT table and locates the private address of the machine that originated the request. It can then return the incoming packet to the right machine, without that machine needing direct Internet access or a public IP address. This NAT system is used by Microsoft’s Internet Connection Sharing system in Windows XP.

The second workaround that conserves the available numbers is to loan a publicly available address only for as long as a connection to the Internet is required. For example, think of an Internet Service Provider (ISP) such as EarthLink. They may have 200 modems available to accept calls from the customers in Seattle. That would mean they need 200 IP addresses available for allocation to each of the 200 modems. Moreover, if they had 200 modems, they probably have 2,000 customers (not all the customers could or typically would want to dial in at the same time)²¹¹. : When a customer connects to a modem at EarthLink, they are loaned an IP address from a server set up to manage this process. That computer is running a Dynamic Host Configuration Protocol (DHCP) server²¹². Like a file server, whose purpose is to hand out files to clients, the purpose of a DHCP server is to hand out IP addresses to clients.

The DHCP server allows the clients to borrow an IP address for a limited time. As soon as the customer hangs up and ceases to require access to the Internet, the unique number is returned (via lease cancellation or expiry) to the available pool of addresses, ready for reuse by another client.

Internet Protocol (IP) 1 Automatic Private IP Addressing (APIPA)