When a machine is configured as a networked system, it must be plugged into the Ethernet with either a twisted pair J45 connector or thicknet 10Base-T connector (depending on the building's Ethernet cable type).

Check the transceiver cable to make sure that you are properly connected to the network. Sun, especially on it's older boxes, is notorious for having bad connections here; try a different transceiver box. It could also be another hardware problem on the network, such as a damaged cable, or a faulty bridge or router.