Apple iMac (21.5-inch, Late 2011) - Manual (Page 52)

52 Chapter 4 Problem, Meet Solution
Network Connections
Make sure the Ethernet cable is plugged into your iMac and into the network.
Check the cables and power supplies to your Ethernet hubs and routers.
If you have two or more computers sharing an Internet connection, be sure that your
network is set up properly. You need to know whether your ISP provides only one IP
address or multiple IP addresses, one for each computer.
If only one IP address is provided, then you must have a router capable of sharing the
connection, also known as network address translation (NAT) or “IP masquerading.”
For setup information, check the documentation provided with your router or ask
the person who set up your network.
An AirPort base station can be used to share one IP address among multiple
computers. For information about using an AirPort base station, see Help Center
(click Help in the menu bar and search for “airport”) or visit the AirPort website at
www.apple.com/support/airport.
If you can’t resolve the issue using these steps, contact your ISP or network administrator.