I am confused and searched a lot, but couldnt find answer to my question, help me out....
ServerSocket s= new ServerSocket(port);
from where does this port number comes from? even if I take any number either of 4 or 5 digits, it works perfectly but why and how? e.g. port number can be any 5009, 12221, 6785, etc. but why? and who decides what number it should be?
It is up to you to specify the listening port for your server side application because the client side has to know to which port it should try connecting to.
As ports range 1-1024 is reserved for system services (this is just by convention, not a rule) you can pick any port from (1024, 65535]. If the port you have selected is occupied by another application, it will just throw an exception. If you do not specify the port number, it will be randomly generated.