To find a public IP address, visit a website such as WhatIsMyIPAddress.com or WhatIsMyIP.com; to find a private internal IP address, run a command on the computer's command line. Private networks use private IP addresses to route data, while computers use public IP addresses to communicate between each other on the Internet.
Websites such as WhatIsMyIPAddress.com and WhatIsMyIP.com detect a computer's IP address whenever the computer connects to the website. A computer transmits its IP address to every website it visits, because this information is necessary to send data back and forth between the website and computer.
If a computer isn't directly connected to the Internet, it likely has a private IP address, which a private network router uses to help transmit data over the local network. Data transmitted to the Internet may pass through a router and modem before reaching it. The modem or router is the device generally assigned a public IP address in a private network scenario, and it is what directs the data transmitted through the network to ensure it reaches the right destination.
Find a private IP address on a Windows computer by running the ipconfig command in a command window. Find the IP address on a Mac OS X or Linux computer by running the ifconfig command in a command window.