Two Machines on Same Modem - One IP address !?!

This isn't actually a Mac question, but all the smart people are here...
I have a cable wireless modem (Linksys) with one (iMac) machine plugged into it via Ethernet, and another (Mac) "attached" via wireless Airport.
When I go to WhatIsMyIP.com (or grc.com, or any other site that detects my IP address) from either machine I see the SAME IP address for both machines.
How can this be? I though addresses were unique. The machines certainly behave as if they were on different addresses - email goes to the right place, &c.

jstackpo,
You are dealing with two things here: The internet as a whole, and the "intranet" that your router creates and manages.
As far as the internet goes, you are correct in the assumption that a single, unique IP address is necessary. However, it is your router, and through the router your entire "intranet," that receives this unique IP address. When the router is powered on, it connects your your ISP's "DHCP" server to request an IP address. When granted, this becomes your "public" IP address, and it is assigned directly to your router. The router then uses a protocol called "Network Address Translation" (NAT) to share that single IP address between all of the connected computers. How does it know to send some information to one computer, and something else to a second computer? Without getting long-winded about it, it knows this the same way your computer knows to send a song you may be downloading from the iTunes store to iTunes, while at the same time sending a Youtube video to Safari.
You can think of your own network (your "intranet") as a mini internet, in that all the computers within it must have unique IP addresses. This same principal applies to your router, as it is seen from other computers on that network. Typically, routers use the IP address range of "192.168.x.x" for the entire network, but there are many choices an administrator can make is this regard. In creating this "private" network, the router acts as a DHCP server in it own right, granting these internal IP addresses as machines connect to the network and request them (just as the router does when it connects to your ISP's DHCP server).
So yes, there are at least three IP addresses involved: 1) Your "public" IP address, requested by your router and granted by your ISP's DHCP server, which applies to your router, and through it every other machine on your network (as they are seen by the internet at large). 2) Your router's self-assigned "private" IP address, as seen by every computer on your network. 3) The additional "private" IP addresses given to the various computers on your network, assigned by your router via its own DHCP server.
Scott

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