I don't much about this..i know little that only iam trying to explain..........
one thing is that local addresses u can't access by using internet...whatever Ip address systems are assaigned when they r connected to lan are all local IP addresses. when any system is connected to lan if any request is came from that system to outside first that address whether is there or not in network it will check.....,if it is there means it's just data transfer between lan devices.....if it is not there means it will give to the default gateway system...this default gateway is connected to the remote server through some global ip by using any ISP ...so,it will send request to that..............
Fisrtly: The two computers on your network should show the same external IP address... You might be behind a transparent proxy and the IP site might show the proxy's IP address rather than your own. http://www.whatismyproxy.com/ does a gooed job of showing your true IP address if the proxies include the proper headers.
Your "default gateway" is used by your computer to contact any IP outside of your subnet (AND the IP and subnet meask to find the network address. All computers with the same network address and subnet mask are on the same subnet)
In most cases the "default gateway" router is a NAT router (in the case of a normal router, your computer will have its internet IP address set on the Ethernet interface)
NAT works by not only forwarding the packets to the correct upstream router but also rewriting the source address to its own address (on the WAN interface, a router always has at least two interfaces and thus at least two IP addresses) and sometimes also changing the source port (TCP connections have both a source and destination port, which usually differ. Mostly source ports are chosen randomly) It then sends all packets belonging to that connection back to the right computer. (Port numbers are used for this)
If your ISP have a transparent proxy server, they intercept all requests on port 80 and forward it to a proxy server ehich does the request on your behalf. This is done to censor data and cache web accesses. Sometimes ISPs has several proxy servers and you can see any one of their IP addresses.
The internet was never designed for NAT and NAT breaks a lot of applications (thats why you need to forward ports on routers) NAT is a trick to let several computers share a single IP, and is necessary because limited number of IP addresses available in IPv4.