DEMARC is the telephone company term for where their facilities or wires end, and where yours begin. When ordering a new DSL connection or new phone service, your local telephone company (for example, Verizon, SBC / ATT.) will install a new pair(s) of wires for the T-1, DSL, and/or POTS lines connection to the DEMARC for your business.

The DEMARC point for business lines becomes more complex, and depends greatly on the type of facility you are in. Most often the DEMARC is the point where the phone company cable enters a building - usually in large black cables containing 25, 50, 100 or more telephone lines. These cables usually terminate in jacks or punch down blocks - the point of demarcation between your wires and the phone company's. This location is normally on the outside of the building, or if you are in an office building, a distribution point on each floor.

With most telephone companies, the DEMARC extension is the customer's responsibility. Not having the DEMARC extension ran, can delay your installation, and operations at your new facility.