Question : Cable Modem IP Addressing

I would like to know how cable modems IP addresses work.
I know that the modem is assigned an IP address. However, WhatI don't know is how the modem (external) comunicates with the NIC card on my PC without setting any IP address on my NIC card. If this question is anserwed then my next question is half answered:
I have two PC and one cable modem. I have connected the two PCs with a hub/router. The router is connected to the Cable modem. Both PCs can access the Internet and work fine. The question is how, when say I go to a site and the site responds to my requests, the site response gets to the right PC which shares the a single IP address of the cable modem.
Please try to explain to me the big picture of how this is done. I need to know the exact addressing scheme.

Thank you.

Answer : Cable Modem IP Addressing

your "hub/router" is performing network address translation (NAT), where incoming and outgoing traffic are translated to and from your single IP address. There are 2 types of NAT...the one used to share a single IP is sometimes called "port NAT".

Each IP packet contains a source and destination IP address. Each TCP or UDP packet also has a port number. Normally the TCP port number is 80 for web traffic.

The NAT program basically builds a table of outstanding IP conversations, and replaces the source port (in outgoing packets) with a random assigned value in a higher range. When the Internet server replies, the destination port number will be the translated port, and the NAT engine will know which system to send (the incoming packet) back to, before re-translating the port back to the original value(80)

Mike