networking - What is a router? What is a switch? and What is a hub?

I always mess up with the basic concepts in networking. This are the very basic questions.

1. What is a router, its functions and what is meant by routing?


2. What is a switch, its functions? Many time I heard that there are layer 3 switches. If there are layer three switches , why they are not called as router? since they are doing work of a router. What is the exact difference by which we decide this router and this is switch?


3. What is a hub?

I have read about these questions plenty of time and then to I have messed up in my interviews. Please let me know or give me a link to study about these things in very detail.

Answer

A hub is the simplest hardware device that is used to interconnect equipments. It provides network ports from which he reads packets and duplicates them to all other ports.

The switch (layer II) is an enhanced hub. The simplest switch can decide on which port to send a received packet so as to not disturb a sub-network where no one is interested with this packet. This is to reduce the traffic collisions.

The router role is to separate two or more networks. Let's say R&D department network, Commercial department network and the internet.

Routing is the action of taking a packet from one network and do something with it (like dropping, relay on another network, ...). Without routing, it is not possible to communicate from one network to another.

Layer III switches are rather like routers but they are used for internal LANs :

1. They have more hardware capabilities instead of pieces of software used in routers


2. Since their purpose is to serve as LAN interconnections, they don't have WAN ports and capabilities (VPNs, etc...).