Demark

In telephony, the demarcation point is the point at which the public switched telephone network ends and connects with the customer's on-premises wiring. It is the dividing line which determines who is responsible for installation and maintenance of wiring and equipment—customer/subscriber, or telephone company/provider. The demarcation point varies between countries and has changed over time.

Demarcation point is sometimes abbreviated as demarc, DMARC, or similar. The term MPOE (minimum or main point of entry) is synonymous, with the added implication that it occurs as soon as possible upon entering the customer premises. A network interface device often serves as the demarcation point.

Prior to the Bell System divestiture on January 1, 1984, American Telephone & Telegraph Company (AT&T) through its Bell System companies held a natural monopoly for telephone service within the United States and Canada. AT&T owned the local loop, including the telephone wiring within the customer premises and the customer telephone equipment. A similar arrangement existed with smaller, regional telephone companies such as GTE.

As a result of deregulation of the telephone system, unbundling of the local loop, and lawsuits by companies wishing to sell third-party equipment to connect to the telephone network, there was a need to delineate the portion of the network which was owned by the customer and the portion owned by the telephone company or the common carrier. Where the portions meet is called the demarcation point.

The demarcation point varies from building type and service level. In its simplest form, the demarcation point is a junction block where telephone extensions join to connect to the network. This junction block usually includes a lightning arrester (which requires a wire to ground). In multi-line installations such as businesses or apartment buildings, the demarcation point may be a punch down block. In most places this hardware existed before deregulation.

...
Wikipedia