If you’re unable to access the Internet on the wired network, first check the network cable connection.
For the wireless network, please first verify your AirPort icon should be displayed as on the menu bar.
Next to check your computer has been assigned an IP address:
1. Select the Finder icon (on the Dock)

2. From the menu bar, click on Go and then Utilities

3. Next run Network Utility application

4. Check that your IP address is in the form of 155.69.x.x or 172.x.x.x under the Info tab as shown. Your link status should show Active too. If your link status shows Inactive, please check your network cable connection or AirPort connection again.

5. If the IP address is in order, you may want to perform a ping test. To do so click on the Ping tab. Enter url or web address (www.yahoo.com) and click on Ping. You should see the reply messages as shown.

6. Successful use of ping indicates that the IP configuration of your system is probably functional.