Apple Mac Pro (Early 2008) - Manual (Page 77)

Chapter 4 Problem, Meet Solution 77
Network Connections
Make sure the Ethernet cable is plugged into your computer 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 if your ISP provides only one IP address
or if it provides multiple IP addresses, one for each computer.
If only one IP address is used, 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. You can use an AirPort Base Station to share one IP
address among multiple computers.
For information about using an AirPort Base Station, search for “AirPort” Mac Help or
visit the AirPort support website at www.apple.com/support/airport.
If you cannot resolve the issue using these steps, contact your ISP or network
administrator.