To avoid too much techno-babble, I'll try and keep it as simple as I can.

Basically, an IP address is the same as a Street Address. More often than not, when you connect to a different network, your IP Address will change. How the server reacts to that, depends on the setup. It sounds like they have allowed a specific address, but whether that is an internal or external address is also applicable. (An internal IP is the address of your Laptop/PC within the local network, and an external IP is the address of your whole network, that is exposed to the internet.

You can identify what your IP is easily by following these steps:

  • For an internal IP:

    1. Open a "Command Prompt" by right-clicking on the start menu, and choosing it from the pop-up menu, or by searching for it by typing "cmd" into the search.
    2. Once open, type "ipconfig", then press enter. A whole lot of info will appear, but you are looking for a line that says: IPv4 Address...... This will show you your laptop's internal IP.
  • For an external IP:

    1. Open your internet browser, and go to google, then type in "What's my IP". This should bring up a response with your external IP, which is the IP for your external network. Now, in the case of being on the same network as the server, this will be less useful, but if you are on separate networks, any device with that external IP will be allowed on the server, regardless of the internal IP.

Adding these IPs to the server list will then allow you to connect.