The internal, LAN-IP address is normally set to a default, private number. Linksys routers, for example, use 192.168.1.1 for their internal IP address. D-Link and Netgear routers typically use 192.168.0.1. Some US Robotics routers use 192.168.123.254, and some Belkin and SMC routers use 192.168.2.1. No matter the brand of router, its default internal IP address is listed in the manufacturer's documentation. Administrators have the option to change this IP address during router setup or at any time later. Unless someone manually changes it, however, this private LAN-IP address remains fixed. This address can be viewed and changed from the router's administrative console.
The external, WAN-IP address of the router is set when the router connects to the Internet service provider. This address can also be viewed on the router's administrative console. Alternatively, the WAN-IP address can be found by visiting a Web-based IP address lookup service like http://checkip.dyndns.org/ from any computer on the home LAN.
Another way to identify the public IP addresses of routers, involves executing a ping or traceroute command. From inside a home network, the (DOS) command "ping -r 1" will send a message through the home router that will cause its IP address to be displayed. For example, "ping -r 1 www.yahoo.com" should result in a message like the following displayed on the command prompt:
Reply from 18.104.22.168: bytes=32 time=294ms TTL=56
On corporate networks, network discovery services based on SNMP can automatically determine the IP addresses of routers and many other network devices.See also - How To Find An IP Address