This simple ethernet tester will check the continuity of a line, similar to portable a ping tester. You don't have to carry over a laptop just to ping the connection.

Get:

• AUI 10BT Tranceiver (an old Mac adapter?)
• small 9V battery
• 9V battery wire conneciton kit (from Radio Shack?)
• 1 ft Category 5 Ethernet patchcord

Do:
 

AUI (PC) AAUI (Mac) Connect Pin 6 -> NEG of battery using 3in of copper wire  Pin 13 -> POS of battery using 3in of copper wire Connect  pin 4 -> NEG of battery using 3in of copper wire   pin 3 -> POS of battery using 3in of copper wire

(See here for a more detailed version. Thanks to Mr.Thomas F. Cook)

• Connect one side of patchcord to Transceiver
• Connect the other side of patchcord to Ethernet drop
• If, "Link" lights up, then the connection is active. When you are done, disconnect the battery to save it from draining its charge. (Or insert a "normally open" push-button doorbell type switch to the circuit)

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Problem:
Have you ever looked into an IDF and scratched your (few/grey) hairs trying to find which port on the-switch is connected to a problem workstation? Here is a new use for the tester described above.
Scenario:
A computer is not connecting to the 'net. You found that the Computer/patch-cable combination at the workstation end works, by trying both on another nearby wall jack. On opening the door to the IDF, you are unable to trace quickly the CAT5 cable that connects from the patchpanel to the switch.)

Solution:

• Detach the patch cable from the non-working jack off the patch panel (at the IDF)
• Attach this end of the patch cable to the AUI and turn power on/off several seconds at a time.
• Watch the LED link lights on the switches for on/off corresponding to your power on/off motions.