Without any doubt one of the best network scanners out there is Nmap, is so versatile that you are only limited by your own knowledge. Despite it’s greatness it remains a no go zone for some people because they have to use the command line. Fortunately, they have the option of using Zenmap which is a GUI for Nmap.
It’s not just another GUI, but one that can also create a topology map of the hosts you are scanning.
There is no point in me describing Nmap since you can visit the sites below to get an overview of what it does.