It depends..... If your MAC is getting an address in the range of 10.x.x.x, 192.168.x.x then some device between you and the Internet is doing Network Address Translation (NAT). The device could be an Airport or Cable Router.
The concept of NAT can be confusing but essentially the router WAN interface gets the Internet address from your ISP and gives your MAC a different address. A feature called Port overloading (PAT) enables the router to share 1 Internet address with multiple computers behind the router.
To allow incoming connections (Web) to your MAC you must tell the Router(NAT) which IP address to send the WWW requests to. (As you may have multiple computers) The Internet only sees the real ISP address of your network, not the other addresses behind the router.