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Old 09-02-2005, 03:36 PM
E. Axel Larsson's Avatar
E. Axel Larsson E. Axel Larsson is offline
Join Date: Jun 2005
Location: Madison, NJ
Posts: 303

Originally Posted by Ginny Palmieri
Ok - I confess my understanding of how a machine grabs an IP address is shaky at best, but I did think it was sort of constant per log-in, at any rate. So, how come, when I go to whatsmyip through Mozilla I get a different response than I do when I go through IE two seconds later?

Illumination continues to be my goal....
What you are seeing when you use one of those sites is the public “real” internet address that your computer is accessing the site through at that moment.

If you run ipconfig from the command prompt, you'll see your computer's current internal IP address. It used to be the case the Drew had internet routable addresses for all of the computers on campus. However, as the IP address space has become more scarce, and when we switched internet providers, we had to start using NAT (Network Address Translation).

With NAT, PCs on the network are assigned internal IP addresses that are not routable on the Internet. We have a pool of "real" Internet IP addresses that are shared amongst all clients on the network and that are managed by our Internet firewall. When your PC tries to connect to a site on the Internet, the firewall dynamically maps your internal address to some address/port combination in this public pool, and maps the response back to your PCs internal IP address. The mapping is dynamic and can change from request to request. NAT allows us to use a much smaller range of real Internet routable addresses compared to the size of our internal IP address space.

This is basically a grown-up version of what happens at home if you use a home networking router with a cable or DSL provider. The provider only assigns your one real IP address as part of your service. The router uses NAT to enable all of the PCs on your home network to share this address.
E. Axel Larsson
Systems Architect and Director of the Enterprise Technology Center

Last edited by E. Axel Larsson : 09-07-2005 at 09:59 AM.
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