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

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Internet Protocol (IP)
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Automatic Private IP Addressing (APIPA)
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Network Address Translation (NAT)

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.

Gateways

What is a gateway? Quite simply, a gateway is a device (normally a router) that allows you to cross from one network to another. Just as a real gate sits in the middle of a property boundary, accessible from both sides and allowing people to pass through, a network gateway is a device that sits between 2 networks allowing data to pass through. By configuring a “default gateway” on a client machine, you are telling it where to send any data that is not destined for a machine on the same network.


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.


 __________________

211. The ratio of the number of modems compared to the number of potential users is commonlyreferred to as the “contention ratio”.

212. Both the physical hardware and the software providing the service are referred to as “servers”. Context is all important!

Previous Topic/Section
Internet Protocol (IP)
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Pages in Current Topic/Section
1
Next Page
Automatic Private IP Addressing (APIPA)
Next Topic/Section

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