From Mac Guides
An IPv4 address is a unique set of numbers given to every computer on a network, analogous to the number of your telephone on a telephone network. This number is a four-byte integer and is usually represented in decimal with a period separating each byte (i.e. 192.168.0.0). An IP address is a logical address usually assigned by a router (a machine may also choose its own IP address). MAC addresses (which physically identify a network interface) are mapped to an IP address when an IP address is used.
There are many IP addresses which are set aside for special uses and which may not be assigned to other uses. For example, the IP address of 127.0.0.1 always points to your computer.
An in-depth description of just how the IP address system works is available in the Wikipedia article on IP Addresses.
Finding the IP Address on Your Mac
Using the Terminal
Open the Terminal application, located at /Applications/Utilities/Terminal, and type “ipconfig getifaddr en0” for an ethernet connection, or "ipconfig getifaddr en1" for wireless, and press Return.
Using the Network Utility
Start the Network Utility application, located at /Applications/Utilities/Network Utility. Your local IP address can be found on the first page.
If you are part of more than one local network, additional IP addresses can be found by selecting the appropriate Network Interface.
Finding the IP Address on Your PC
- Click Start, then Run, and enter "cmd", and click OK. In the DOS-like window that appears, type "ipconfig” and press Enter.
Finding Your External IP Address
- Each connection to the internet is assigned its own unique IP address which defines it as a part of the global network.
- Websites such as WhatisMyIP, myIPddress.com and IP Chicken provide a quick and easy way to determine your external IP address.
Troubleshooting IP Addresses
- If you're behind a network, your IP address will probably begin with 192 or 10. These are valid numbers. However, if it begins with 169, or is 0.0.0.0, then there is a problem with your network configuration.
- If you connect through a router, you often need to connect directly to the router itself. Routers usually have IP addresses of 192.168.1.1 or 192.168.0.1.