In computer networks communication occurs between the entities in different systems an entity is anything that is capable of sending or receiving messages. However two entities will not simply send bit streams to each other but expected to be understand those bit streams, for communications to occurs the entities must agree on a protocol. A protocol is a set of rules that govern data communications. A protocol defines what is communicated how it is communicated and when it is communicated.
The exchange of messages and the actions takes when these message sent and received are the key defining elements of a protocol.
A protocol defines the formal and the order of messages exchanged between two or more communicating entities as well as the actions taken on the transmission and or receipt of a message or other event.
As an example of a computer network protocol with which one is probably familiar consider what happens when one will make a request to a web server that is when one type the URL ( Uniform Resource Locator) or a web page into the web browser. First the computer will send a connection request message to the web server and wait for a reply. The web server will eventually receive the connection request message and return connection reply message. Knowing that it is now ok to request the web document. The computer then sends the name of the web page which is needed to fetch from that web server in a GET message. Finally the web returns the web page to the computer.
The internet and computer networks in general make extensive use of protocols. Different protocols are used to accomplish different communication task.
Protocol are divided into two categories based on their use of connections.