Sharing a modem INTERNET on LAN

You need a router. You can configure your computer to act as a router (this is usually referred to as "connection sharing"); most OSes have some built-in mechanism to do this.

Or you could just buy a hardware router, and put it between your modem and your computer. This is probably both easier to set up and more flexible (as it will work even if your computer is off). Simple hardware routers (even with WLAN built in) are quite cheap, usually starting at about 30 $ / Euro.

Now, I want to share this modem internet to one of the ip address available on LAN say 10.100.99.56..

Normally, you'd make the internet connection available to all computers on the LAN. If you want to restrict it, that can usually configured on the router as well.