hi,
I prepare for a project for LAN monitoring,and I want to display hosts connected to the network with a jTree( the name or the adress of the machine),I do not know where I can start,is there any open source software that allows making something similar and thanks for help

Its a simple project but with network understanding. For Lan Monitering you must be connected as a gateway so you can map p.c's which are attached with network. For starter search api's which can help you getting connected p,c information. Please watch these api's
http://jnetpcap.com/
, arranging them as a jtree node after getting p.c addresses is v easy...