我应该使用什么端口号？

Question

I am writing an app that is to be running on a Windows PC. I need to create a server socket listening on 127.0.0.1, and another client socket which is to connect with this server socket.

Since the data exchange between the two sockets are within the same machine and there is no client connecting from outside of the machine, what port to use is insignificant, as long as the two sockets use the same port number.

So, how do I decide which port number to use ? Shall it be a hard-coded port number such as 49500? What if another unrelated app on this machine happen to use this port number? Or shall I get the list of all used ports and programmatically pick an unused port?

Just want to know what is the best approach. Thanks.

Answer 1

ports within 0 to 1023 are generally controlled and you should assign your socket with higher port numbers, although in that range ports within 1024 and 49151 can be registered for others to be informed about that and not use them.
if you want to avoid conflicts you can see registered ports on your machine and assign a port number to your socket which is empty but ports higher than that (49152 to 65535) are completely free and are not even registered.
generally, it is not common to worry about that. for example, two major applications like VMware and apache web server operate on the same port number (443), and if you want to use VMware workstation and Xampp (which works with apache) you have to simply make one of them listen on another port and its not a big deal. so in my opinion the best practice is to let your users change this via a config file or something similar.
for further information, you can search google. for instance this link might be useful:
https://www.sciencedirect.com/topics/computer-science/registered-port#:~:text=Well%2Dknown%20ports%E2%80%94Ports%20in,1023%20are%20assigned%20and%20controlled.&text=Registered%20ports%E2%80%94Ports%20in%20the,be%20registered%20to%20prevent%20duplication.&text=Dynamic%20ports%E2%80%94Ports%20in%20the,assigned%2C%20controlled%2C%20or%20registered .