I have configured a simple LAN using Ubuntu router. We know that if we don't impose any restriction, total bandwidth of Internet will be distributed among LAN user.
If any user try to get half of my total bandwidth, Is it possible?
Yes. Essentially, the total bandwidth will be distributed amongst all competing users, so if someone has an application that talks enthusiastically enough to the internet, and if noone else is doing so, she'll get all the bandwidth.
How things are decided if the bandwidth is contended is a long and complex issue, but the practical upshot is that unless you decide to prioritise otherwise, two equally-competing users will likely end up sharing the bandwidth 50:50; three competing users, 33:33:33, and so on. Not all competing users will be equally-competing.
If you want to prioritise some users, or classes of traffic, over others, I recommend close study of