The firewall on a local area network is responsible for blocking unwanted traffic from entering or leaving the network. Part of the firewall's job is to perform NAT--network address translation--to allow all of the computers in the network to access the Internet at the same time. Servers running inside NAT environments are difficult to connect to because they cannot receive traffic directly; clients must send their traffic to the firewall instead.
Log in to one of the computers in the local area network that is protected by the firewall.
Open a Web browser.
Type in the URL of one of the many free address-checking services. (See Resources for examples.)
Connect to the website.
Read the page and find your Internet protocol address: This is also the IP address of your firewall's public interface.
Firewalls have at least two interfaces. To find the IP address of your firewall's internal interface, check the default gateway (also known as the default route) on the computers behind the firewall. On Windows XP, for example, use the "route PRINT" command and check the "Gateway" column for the "Network Destination" of 0.0.0.0.
The IP address returned by address-checking services is not necessarily accurate. If your Web traffic is redirected to a Web proxy outside the firewall, as is routine in corporate networks, you cannot trust the results of the address-checking services.