Demarc and NIU

Demarc
Connections from the outside world—whether network or telephone—come into a building at a location called a demarc, short for demarcation point. The term “demarc” refers to the physical location of the connection and marks the dividing line of responsibility for the functioning of the network. You take care of the internal functioning; the person or company that supplies the upstream service to you must support connectivity and function on the far side of the demarc.
In a private home, the DSL or cable modem supplied by your ISP is a network inter face unit (NIU) that serves as a demarc between your home network and your ISP, and most homes have a network interface box.