Let’s say that a computer cannot access the Internet. Here are a series of steps to discover where the problem lies.
 Does the computer have an IP address?
In Windows 2000 or XP, click Start, then Run. Type cmd and click OK to open a command window. At the command prompt, type ipconfig and press enter. If the TCP/IP address that appears starts with 169.xxx.xxx.xxx, the computer does not have a routable IP address. In Mac OS X, check the network control panel in System Preferences.
 Possible solutions: Check network cables to make sure they are plugged in. Try restarting the computer.
 Can the computer communicate with others on the network?
Identify the IP address for the router, using the ipconfig command. At the command prompt, type ping 192.168.0.1 (or whatever the IP address is for the router) and press Enter. If replies come back, then network communication is working. If not, something is not right. To open a command window on Mac OS X, open the Terminal application, in the Utilities folder, which is in the Applications folder.
 Possible solutions: Check network cables to make sure they are plugged in. Restart the router in case it has crashed.
 Can the computer ping an IP address on the Internet?
Perform the same step as above, but use an IP address outside the organization. One example is 216.239.63.104, which belongs to a Google web server. If replies come back, the router is passing packets back and forth between your network and the Internet. If not, something is wrong with your Internet connection or router.
 Possible solutions: Restart the router and the modem.
 Can the computer resolve names to IP addresses?
At a command prompt type nslookup www.google.com. The computer should return the IP address of the web server. If it doesn’t, then something is wrong with the DNS system.
 Possible solutions: Check DNS entry in the TCP/IP settings. Restart the router. Contact your ISP to ask if their DNS server is down.