How can I put my program behind a firewall or make it more secure?

Question

I have a client server program and I was reading through my notes. I had jotted down a recommendation to put it behind a firewall. However, I do not know how to do this in Java at all or even where to begin. How do I even create a firewall in Java?
I'm also not sure if this is the security I need.
This program is already running inside a network limited only to certain users. However, this particular program is limited only to two users. Also, the client runs on Unix and Windows machines accesible by many users in our project. I would not know how to create a firewall for just this program and just these two users.
Right now my program is sorta like the basic client/server program examples given in the Java tutorials. It verifies the IP address of the client but like I said before anyone can be logged into that IP but only two users should be allowed to run the commands.
Would a firewall even work in this case?

Answer 1

However, I do not know
how to do this in Java at all or even where to begin.
How do I even create a firewall in Java? You don't.
I'm also not sure if this is the security I need.I don't think so.
I would not know how to create a firewall for just this
program and just these two users.A firewall is not for restricting access to particular users of a network. It's for keeping unauthorized people out of the network entirely. What you should do is secure the server (how to do this depends on probably many factors), and use a username and password in the client app to control access. The client contacts the server, passing the credentials, and the server either grants or denies access. It's possible you would want to hash and salt the password, depending on how secure it needs to be.