how to find out computer name by ip LAN

Try using the command nbtstat -a [ip address] in the command prompt:

win+r then type cmd and then press enter.

This should display a table of names. The name associated with the type "Unique" is the name of the computer. The name associated with the type "Group" should be the workgroup the computer is joined on.

The kitchen sink answer is to just run nmap against the IP.

That'll get you just about all the information it's possible to acquire from probing the computers network interface.

While it's a bit overkill here, nmap (and the GUI included - zenmap) are tremendously usefull in and of themselves.