What is a NID?
A network interface device (NID) is the small box, usually on an outside wall of a home or in a building's utility room, where the telephone company's line ends and the customer's inside wiring begins. It marks the demarcation point (demarc): the provider looks after everything up to and including the NID, and the customer or building owner is responsible for the wiring after it.
What's inside
- Provider side: the incoming line from the street, usually sealed so only the telco opens it.
- Customer side: terminals for the inside wiring and a test jack (an RJ11 port) for each line.
- Surge protection: a protector that sends lightning and power-line surges to ground before they reach the inside wiring.
Using the test jack
To find out whether a phone or DSL problem is the provider's or the customer's, disconnect the inside wiring at the NID and plug a known-good phone, or the DSL modem, straight into the test jack. If it works there, the fault is in the inside wiring or equipment. If it doesn't, the problem is on the provider's side.
NID vs. NIC
Don't mix them up. A NIC (network interface card) is the adapter in a computer that connects it to a network. A NID is the provider's demarc box on the building. Fiber service uses an ONT (optical network terminal) as the equivalent hand-off point.
For the CompTIA A+ exam
Know the NID as the demarcation point between the provider's network and the customer's, and that troubleshooting a home or small-office phone or DSL line can start with a test at the demarc.