Is There Anything In The Router..

Is there anything in the router that doesn't change, and is unique to everyone/most people that java can read? I'm using an authentication system and I would like to use something like this. I was using internal IP but it changes to much. Thanks :)

EDIT: or even on the computer.

MAC Address is the only thing that comes to mind unless you set up static IPs. However, MAC Address spoofing will most likely make this a less than secure Authentication system.

Unless you move into advanced networking, there is very little that doesn't change. MAC Addresses don't change, but can be spoofed. If you set up static IP's that could help, but depending on the configuration someone could simply change that too. Why are you trying to make a security system based off of a router's set up?

Because, it's easy to call and read, and I can't think of anything else in the users computer that wouldn't change and is unique and java can call (preferably in number format)
But I did post this: "EDIT: or even on the computer. "

EDIT: Okay, I did

Code:

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            InetAddress []addresses = InetAddress.getAllByName(InetAddress.getLocalHost().getHostName());

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then I can do addresses[number], but each number is different, how do I know which one to use?

Code:

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                        else if(s1.equalsIgnoreCase(".get")){
                        try{
                                        String desktopPath = System.getProperty("user.home") + "/Desktop";
                                      File f;
                                      f=new File(desktopPath.replace("\\", "/")+"/get.txt");
                                      if(!f.exists()){
                                      f.createNewFile();
                                      }
                                      InetAddress []addresses = InetAddress.getAllByName(InetAddress.getLocalHost().getHostName());
                                              Writer output = null;
                                              String text1 = String.valueOf(addresses[1]).substring(String.valueOf(addresses[1]).indexOf("/")+1)+String.valueOf(addresses[2]).substring(String.valueOf(addresses[2]).indexOf("/")+1);
                                              String text2 = text1.replaceAll(":", "");
                                              String text3 = text2.replaceAll("%", "");
                                              String text = text3;
                                              File file = new File(desktopPath.replace("\\", "/")+"/get.txt");
                                              output = new BufferedWriter(new FileWriter(file));
                                              output.write(text);
                                              output.close();
                        }catch(Exception e){
                                System.out.println("Error");
                        }
                        }

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Hey guys, could you please review that and tell me if it looks fairly safe (I could also remove the letters to make it even more obvious it's not using the MAC address, if it is)?
My authentication works by the user typing in .get, a text file being created on his desktop, he tells me everything in the text file, then I add it to the authentication (which I already have 100% worked out, as before I was using internal IP).
Though I may later change it to when he types .get it sends it to a MySQL database, so he never sees the number anyway.