This is a commonly-used network protocol. The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. X.25's protocols correspond closely to the data-link and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.

application layer provides services for an application program to ensure that effective communication with another application program in a network is possible. The application layer is not the application itself that is doing the communication. It is a service layer that provides these services:

    * Makes sure that the other party is identified and can be reached
    * If appropriate, authenticates either the message sender or receiver or both

There are 7 Layers of the OSI model which are:-

1. . Application Layer (Top Layer)

2.  Presentation Layer

3.  Session Layer

4.  Transport Layer

5.  Network Layer

6.  Data Link Layer

7.  Physical Layer (Bottom Layer)