It will monitor all the machines on your network if you set it up to do so. It's not something you just install and flip a switch and everything works. You have to put a client on each machine for detailed monitoring, and you have to configure an entry for each machine in nagios. Also, I wouldn't recommend messing with getting Solaris up and running just to put nagios on it, especially since Sun is no more and who knows what Oracle will do with Solaris now. Just try something easy, like ubuntu server or debian to get it up and running.