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A demarcation point is the physical point at which a telecommunications company's public network ends and the customer's private network begins. The demarcation point is often the point at which the cable physically enters the building, but this varies from one country to another.
The demarcation point defines where the telephone company’s responsibility for maintenance ends and the consumer's responsibility begins. The demarcation point contains a surge suppressor to protect the wiring and connected equipment in a cutomer's home from external or internal damage. It also permits consumers to disconnect from the telephone company's wiring for troubleshooting.
The demarcation point is also referred to as a network terminating interface or demarc.
Demarcs are located on the exterior part of homes in gray or black boxes. They may also be placed in a home's basement. In some apartments, the demarc points are located in main telephone rooms or in the suite behind blank outlet cover plates or utility rooms. Demarc maintenance is the responsibility of the telephone company.
Demarcation points differ depending on the building type and service level. They include a junction block where telephone extensions join to connect to networks. The junction block includes a lightning arrestor. Demarcation points have user-accessible RJ-11 jacks connected to the telephone network, and small loops of telephone cords connecting to jacks by modular connectors.
In the United States, the demarcation point is a device defined by Federal Communications Commission rules designed to provide for the safe connection of third-party telephone customer premises equipment and wiring to public switched telephone networks.
In the U.K., demarcation points occur within a jack whose wiring is partly owned by customers and partly by phone companies. Other jacks are customer properties. The new NTE5 jacks have removable front panels, in which wiring belongs to the customer.