Network Devices and there working

The following icons will be used to represent network devices for all guides

on this website 

Layer-1 Hubs

Hubs are Layer-1 devices that physically connect network devices together  for communication. Hubs can also be referred to as repeaters.

Hubs provide no intelligent forwarding whatsoever. Hubs are incapable of processing either Layer-2 or Layer-3 information, and thus cannot make decisions based on hardware or logical addressing.

Thus, hubs will always forward every frame out every port, excluding the port originating the frame. Hubs do not differentiate between frame types, and thus will always forward unicasts, multicasts, and broadcasts out every 

Layer-2 devices build hardware address tables, which will contain the following at a minimum:

• Hardware addresses for host devices

• The port each hardware address is associated with

Using this information, Layer-2 devices will make intelligent forwarding decisions based on frame (Data-Link) headers. A frame can then be forwarded out only the appropriate destination port, instead of all ports.

Layer-2 forwarding was originally referred to as bridging. Bridging is a largely deprecated term (mostly for marketing purposes), and Layer-2 forwarding is now commonly referred to as switching.

Layered Communication:--

Network communication models are generally organized into layers. The OSI model specifically consists of seven layers, with each layer representing a specific etworking function. These functions are controlled by protocols, which govern end-to-end communication between devices.

As data is passed from the user application down the virtual layers of the OSI model, each of the lower layers adds a header (and sometimes a trailer) containing protocol information specific to that layer. These headers are called Protocol Data Units  (PDUs), and the process of adding these headers is referred to as encapsulation.

The PDU of each lower layer is identified with a unique term:

For example, switches are generally identified as Layer-2 devices, as switches process information stored in the Data-Link header of a frame (such as MAC addresses in Ethernet). Similarly, routers are identified as Layer-3 devices, as routers process logical addressing information in the Network header of a packet (such as IP addresses).

However, the strict definitions of the terms switch and router have blurred over time, which can result in confusion. For example, the term switch can now refer to devices that operate at layers higher than Layer-2.