port 81

This port 81 is used by some 'alternative' webservers like light-http,nginx,etc.
Since it is very close to http port in case of default http server (IIS,apache) is installed
on computer, this 81 port is used by these lightweight webservers.
You can check if this port is open by porttester , specify remote computer and 81
as a port.
If porttester will return 'Port available' most likely this port is used by some webservice.
Open task manager and try to look after processes like lighttpd.exe or nginx.exe,
shutdown this process and recheck the port, if porttester will return Port is not available
that means this port was using by this webservers.