SSDR and 6400 - public firewall setting on v1803

This may be old news to some but this morning SSDR (.33) was not seeing my 6400; it was fine last night.   Earlier I made some changes in the sharing setting to network to printers, but I do not recall touching the firewall settings though advanced sharing does have a Public settings section. Sharing in the Public profile has printers and files, and network discovery are off.
After ten minutes of "oh no!" I turned off all firewalls (Domain, Private, Public) and voila the 6400 was connected. Then turned each back on in sequence and found the Public firewall when turned on disconnected the 6400. 
The solution was in the "allow an app thru the firewall" setting.  SSDR shows up twice in the list, probably as intended, but the box for public was not checked.  I checked the box, turned on the Public firewall and the 6400 remained connected.  This option does pop up during a new install of SSDR - private is allowed, public is not, apparently by default.
Paragraph 37.1.2 gave me a clue regarding the firewall though it does not mention the Public firewall.  My default is to never allow an app thru the public firewall on a computer; inbound or outbound without good reason. 
The 6400 is direct connected to the computer with SSDR.
Why would I need SSDR as an allowed app through the Public firewall for the 6400 to direct connect to SSDR? This is W10 v1803.  I do not recall this issue in v1709.

This is probably a case of, "did you touch it??" but nevertheless I got bit.