I have a small LAN and one of the PCs (Windows 10) started to behave weird after a Windows update.
Here it how it behaves:
This PC can be seen on the network. If I ping it from other computers, it works.
The shared folders on this PC can be accessed by other computers correctly.
But programs on this PC that must respond to requests made by programs installed on other devices with access this PC in either ethernet or wifi by [IP:Port#] , now don't work anymore. These programs used to work properly before and I think the change in behavior happened after a Windows update.
An example of such program is "Winpopup LAN Messenger" ( https://www.softpedia.com/get/Internet/Chat/LAN-Messaging-Clients/Winpopup-LAN-Messenger.shtml )
I checked the network advanced sharing settings and it's set like before (Turn on network discovery, Turn on file and printer sharing).
I disabled the firewall / windows defender.
With netstat -an command in cmd prompt I can see that those programs are listening to the UDP ports that they are supposed to listen to, however they don't respond to incoming requests from other machines.
I am really puzzled about this weird new behavior.
Any hints would be really appreciated.
I could figure it out. All the problems came from the firewall settings.
In Windows Defender Firewall > Allow an app or feature > check both 'Private' and 'Public' for all instances of WinPopup LAN Messenger > click OK Also, the port used is 1152 and protocol UDP. This port must be opened (also in firewall).