• The first computer networks were designed with the hardware as the main concern and the software as an afterthought.

• This strategy no longer works. Network software is now highly structured.

• To reduce their design complexity, most networks are organized as a series or hierarchy of layers or levels.

• The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network.

• Layer n on one machine communicates with layer n on another machine on the network using an some rules known as the layer n protocol.

• A protocol is an agreement between the communicating parties on how the communication is to proceed.

• The entities comprising the corresponding layers on two communicating machines over the network are called peers.

• In realty, no data is transferred from layer n on any two machines. Instead, each data and control information is passed to the layer below.

• Additional information including protocol control information may be appended by each layer to data as it travels from higher to lower layers in the form of layer headers.

• Below layer 1 is the physical medium through which actual communication occur over communication channels.

• Between each pair of adjacent layers there is an interface.

• The interface defines which primitive operations and services the lower layer offers to the upper layer.

• The set of layers and associated protocols is called network architecture.