Types of Protocols
Just as two people can communicate successfully through the use of a single language, two computing devices can network effectively if communication is governed by the same protocol. Specifically, communication between people is accomplished through a number of norms, including accepted rules or expectations about when to listen, when to speak, appearance and acknowledgement. Likewise, computing devices need a set of rules that, when followed, result in successful network communication and data sharing: This is the purpose for protocols.
Communications (Network) Protocols
Communications protocols are the sets of rules by which communication over a network is achieved. Communications protocols are responsible for enabling and controlling network communication: They set the rules for the representation of data, the signals used in communications (e.g., signals regarding how the connection will be established or how information is exchanged), the detection of errors, and the authentication of computing devices on the network.
Routers are the hardware devices (with their own software) that physically allow computing devices to connect to a network. Each connection point is called a node. To be able to transmit packets of data between nodes via a network, the routers must be able to communicate and, thus, require their own protocol. Routing protocols allow the routers to determine which path to use when transmitting data over a network. Routed protocols handle the actual transmission of those packets once the path is established.
A protocol suite is a deliberate combination of protocols (each with a specific purpose) that, when combined, define how certain media will connect and transport data across a network. The protocol suite is made up of the protocol stack---the defined set of protocols amassed in hierarchical layers according to the level of communication each protocol is responsible for (low, medium or high). Generally the protocol stack features lower-level protocols that set the rules for the physical interaction between networking hardware devices and higher-level protocols, with their advanced features, that set the rules for user applications.
Protocol layers serve as building blocks of the protocol stack. At the base of the stack (the lower layers) lie the lower-level protocols. They generally define how media and hardware devices, such as switches and routers, communicate. Other lower-level protocols tackle methods for networking with the Internet. For example, in the Internet Protocol Suite, the lowest layer is the link layer; it consists of protocols that define the physical transmission of data. The next lower-level layer of the suite, above the link layer, is the Internet layer, which contains protocols that specify how data packets are to be transported from the host, which originates the data, across the Internet network and sometimes to the destination.
At the top of the stack (the highest layers) are the higher-level protocols. These protocols most often define end-to-end communication or are used to transmit data packets to their appropriate destination---the applications found on the host devices and computers connected to the network. High-level protocols also feature their own system of addresses that they use to identity other high-level protocols when wishing to transmit application data. The features of these higher-level protocols are represented, respectively, by the transport and application layers of the Internet Protocol Suite.