The Interface Info tool allows you to view address information and statistics for each IP interface. Use the popup menu to select the corresponding network device.
Many home or small office networks share a single public (globally unique) IP address using some form of Network Address Translation (NAT) that assigns private IP addresses to local hosts. When the Interface Info tool is opened (or "Update" is pressed), the tool tries to determine this public IP address by querying http://checkip.dyndns.org/. If you are behind a NAT router, this public IP address will be different from the IP address of your primary interface, the first active interface listed in the Network Interface popup. If you are running a network service behind a NAT router using inbound port mapping, the public IP address is the address remote clients would use to contact your server.
The Interface Info tool also shows the Link Speed for Ethernet or AirPort devices, the Link Status, and transfer statistics which are useful for debugging local connectivity problems.