https://en.wikipedia.org/wiki/Virtual_Network_Computing#:~:text=VNC%20by%20default%20uses

%20TCP,Java%2Denabled%20web%2Dbrowser.

VNC by default uses TCP port 5900+N,[6][7] where N is the display number (usually :0 for a physical

display). Several implementations also start a basic HTTP server on port 5800+N to provide a VNC

viewer as a Java applet, allowing easy connection through any Java-enabled web-browser. Different

port assignments can be used as long as both client and server are configured accordingly. A HTML5

VNC client implementation for modern browsers (no plugins required) exists too.[8]

The default port a VNC Client uses to connect is TCP 5901. This port corresponds to the first display

number (:1) of the VNC server. Each display number has its own port number, starting from 5900. For

example, display :2 uses port 5902, display :3 uses port 5903, and so on. The user can specify a

different

port

number

if

needed.

Reference:

https://documentation.suse.com/sles/15SP3/html/SLES-all/sec-vnc-connect.html