That all depends on the service you buy from your ISP. If you have a dailup connection your IP address will vary, depending on which of the ISP's modems answer your call. If you have broadband you can generally get afixed IP address. I work for an ISP and all our broadband customers now have fixed IP addresses. Your ISP may do things differently. Sometime your IP address will change each time you get disconnected (i.e, randomly every few hours or days)There are companies (try
http://www.dyndns.com/) that will help you get, or at least appear to have, a fixed IP address. What they actually do is reserve an IP number for you and then redirect any connections sent to that IP address to whatever your ISP is giving you at the time, keeeping track of the changes. You run a program on your mac that calls dyndns every time you connect and basically says "I am not available on 123.123.123.123 (not a real number). Please forward any calls for me there."