Use a little freeware tool from Paessler to show the list of IP addresses that are configured on your PC and can be used for the internal webservers of PRTG Traffic Grapher and IPCheck Server Monitor.
A quick way to check your installed local IPs is using this tool:
download.paessler.com/download/iplist.zip (200 kb)
It lists all installed IPs available on your local PC.
All these IPs can be used for the internal webservers of PRTG Traffic Grapher and IPCheck Server Monitor.