why we can't share internet connection using switch or hub?

[johntrottier]

Well, some of the answers you got were technically correct, and some were just rubbish.
The simple way to answer your question is that a router is a device that is designed to share an address on one network (your ISP's) with many devices on a second network (your home network or LAN).
A switch (or hub as they insist on calling it across the pond, much to the confusion of us colonials) is a device to connect multiple machine to the same network. It cannot span networks.

So to make things simple for the average consumer, whose knowledge of networking usually starts and ends with the term "Ethernet", manufacturers have bundled several devices into one box, called a "home router", which may have some, or all of the following devices

1. A modem - to connect from one type of media (DSL) to another (Ethernet)
2. A router - to connect the home network to a providers network
3. A switch - so the consumer can connect more than one device
4. A DHCP server - so the consumber does not have to assign IP addresses
5. A radio transmitter/receiver - so you can use wireless to connect to the Internet instead of wired cables
6. A little web server and applet - to hold the setup program

All in one box - grab a time machine, take this back to the days of ARPANET and they would freak