How to implement computer-to-computer connection in JAVA?

Question

Java

Hello! I want to implement a computer-to-computer connection. Level - self-taught. Let's say I have a program (a game, for example, tanks, tic-tac-toe), designed for two people.

How can I implement the relationship between these two computers that have dynamic ip addresses. I read that this requires an external server.

What should it be like if, for example, the game is designed for two people, and the game was launched by more than two. Need to ensure interconnected pairs of client computers do not conflict with each other?