Demarc is a short form of containment, which is basically a physical place where the customer and service provider liability is marked. In terms of the public who demarc is the point where the responsible service ends. Anything on the network side was responsible for telecommunications features and above all it is not.
first telephone network was wholly owned by the service provider. All of telephone equipment until the core infrastructure, the telecom provider delivered and maintained. In this homogeneous network had no idea demarc at all.
However, with unbundling, there were many third-party equipment and devices that were presented in the network. Now then, they were not put a network provider, they did not take ownership of them. This is where there was a need to introduce a demarcation point. The idea was that up to demarc be provided and designed by the telecommunications provider and would fall under their scope.
The demarc promotion allowed customers to customize their intranet as per their requirements. They had only to interface with the telephone provider in the standard interface to access the service. Anything on the client end demarc was their responsibility and that is why they could design what they wanted.
Over the years, although the term demarc has changed in many countries. In the United States, demarc point is called the Network Interface Device which itself is owned by the service provider. It can be in an indoor unit or an outdoor one. The UK demarc points are simple jack with a standard interface. Wiring for this jack is partially owned by the customer and part of service providers.
Finally, demarc is an important concept in modern telephony. There is a simple way to separate ownership of the network between the client and the telecom network provider. Without this idea that there had been some issues in service fulfillment and assurance.