How can I create a TCP connection in Python between 2 PCs

So far I have made a VERY basic client/server application that creates a TCP connection. I have a lot of programming experience, just never did this low-level stuff and especially nothing with networks. Note that all the prints are just to help me figuring out what is going on. One of the known issues is that jsonip sometimes gives me an IPv4 and sometimes v6, I don’t know why but that doesn’t matter for now, just to warn anyone who wants to recreate my code.

Server:

import socket
import requests

port = int(input("Enter port you want to open:n"))
#todo: add errorhandling
print("Adding socket...")
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
hostname = socket.gethostname()
print(f"Hostname: {hostname}")
ip_address = socket.gethostbyname(hostname)
print(f"Host address: {ip_address}")
r = requests.get(r'http://jsonip.com')
public_ip_address = r.json()['ip']
s.bind((ip_address, port))
print("Is open for connections on IP: "+public_ip_address+" and Port: "+str(port))
s.listen(5)
print("Done initialisation, listening for incoming connections...")

while True:
    clientsocket, address = s.accept()
    print(f"Connection from {address} has been established")
    clientsocket.send(bytes(f"You have connected to server: {hostname}", "utf-8"))

Client:

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
ip = input("Enter IP to connect to:n")
port = int(input("Enter Port to connect to:n"))
print(f"Connecting to server {ip} ...")
s.connect((ip, port))

msg = s.recv(1024)
print(msg.decode("utf-8"))

On my local machine: Open 20000 in my server.py, it tells me the host is 127.0.1.1, I then enter 127.0.1.1 into my client script and 20000, and they connect. So the Socket has been bound with the 127.0.1.1. (Side question: What is this IP address, is it like the internal IP address of processes in my PC or something? If running ip a on my other machine it is the first one shown of 2)

Using Virtmanager on my machine and running one Linux Server (command line only) and one normal Ubuntu, the server tells me the host is, again, 127.0.1.1 which I don’t need to enter into the other VM to know it won’t work, what does work however, is getting the IP-address of the Server via ip a, which in this case is 192.168.122.37, and when I enter this IP address into the client, it connects. But in the socket here I bind, again, the 127.0.1.1, so is it arbitrary what I put here? What SHOULD I bind here, the public, the weird or the 192. address?

The first thing I could not get to work was using 2 physical devices. When opening a server on my Linux machine, I cannot connect with my windows machine at all, no matter if I use my public, my 127. or my 192. IP-address. Now my end goal is doing this over the internet so I am walking myself up, describing here the steps I took to try and get where I want to be but here I hit a brick wall where I don’t know what is wrong. Am I binding the wrong address on the server, is my router being a problem, is there something else wrong?

I also tried leaving my network using my friends pc a few countries over, but this also just results in a timeout (my theory is that the Router port he is trying to open is closed and I have now idea how I can make the router send data to his PC, which should be not impossible as firefox and every application using internet does it without me having to manually forward every port, I just don’t know how). This is my end goal, creating a connection between my friends PC and mine, and this is how far I got (I wouldn’t mind skipping the local network if it is not relevant for fixing the global connection problem), so, tl;dr: what did i do wrong, what do i need to bind and what do i need to do for the final result to work?

Answer

There are many questions to answer.

Addresses 127.X.X.X are reserved for the loopback interface, most common one is 127.0.0.1. The loopback is a virtual, but important interface and as you have probably guessed, it is usable on the local machine only. You cannot use 127.X.X.X address to make two hosts to communicate with each other.

Addresses 192.168.X.X (and also 10.X.X.X and 172.16-31.X.X.) are reserved for local LANs. They are not valid on the Internet.

You cannot use these addresses to make two hosts to communicate with each other over the public Internet (unless you create a tunnel, an advanced networking topic)

Almost everybody uses them, because we ran out of IPv4 addresses long time ago, they were difficult to get, expensive, etc. Also such hosts are isolated from the Internet, they can be reached only via a router that translates addresses. Such router feature is called NAT. A typical router has one valid Internet address and all connections to the Internet appear as coming from the router. If you contant a service like jsonip.com from a PC, you get your router’s address, not your PC’s address.

To make your program working, make it to accept connections on all interfaces. See the first example in the socket docs. On Linux, use port numbers >= 1024. Ports < 1024 are reserved, not available to regular users.

Final point is that a firewall may prevent connections to your server. It depends on your system and setup.

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Answer