How can use public IP to see if the owner is online or not and the owner is using a private ip on the network assunming that I know both IP's.
Thanks for your question anmar_hotspot.
Are you talking about a private network such as in a business or about monitoring someone on the internet?
If it's a private network and you have administrative access, there are a number of software solutions that will monitor all network traffic. The easiest way would be through a router that has the ability built in.
If your talking about monitoring someone on the internet to see if they are online or not at any given time, I would have to say no but there could be something out there that I am not aware of.
What Schnerdly said, basically. If this is over the Internet, then you might run into privacy laws that prohibit this kind of monitoring. I'm not a lawyer, but I believe I've seen that this could even be considered a form of stalking in certain situations.
There is all kinds of malware out on the market that includes a kind of "phone home" function, and something like that could be used. CompuTrace even has this built into legitimate software. Sort of a LoJack for PC's. Of course, you have to be the owner of the PC. Instant messengers like Google's IM, or Yahoo Messenger, they can be set to notify you when someone logs into that service. But the person on the other may could simply choose not to log in.
If you're trying to do this without the other person's permission, you're asking for trouble, IMO.