Apple Mac Pro (Early 2009) - Manual (Page 15)

Chapter 1 Ready, Set Up, Go 15
Step 4: Connect an Ethernet cable.
To access the Internet or a network, connect one end of an Ethernet cable to either of
the gigabit Ethernet ports on the back of your Mac Pro. Connect the other end to a
cable modem, DSL modem, or Ethernet network.
To use a dial-up connection, you need the external Apple USB Modem, available from
the online Apple Store at www.apple.com/store or from an Apple Authorized Reseller.
Plug the Apple USB Modem into a USB port on your Mac Pro, and then use a phone
cord (not included) to connect the modem to a phone jack.
Gigabit Ethernet ports