While nettool does interact with the "Internet", I don't think this is the
optimal category for it to exist in.  Internet is more of a general user
software category for things like Email, Web Browser, Instant Messenger and the
like.

Since nettool is geared towards the administrator type user I think the best
category it could be in is the System Tools category.  Of course somthing like
"Network Tools" would be most appropriate, but System Tools seems close enough
that the people looking for this software will be able to find it there.

Perhaps something like this in the .desktop file:

Categories=GNOME;Application;System;Utility;

I would also suggest that the name follow the HIG more closely and be "Network
Information Tools", with a comment of "Analyze and view network information"