I am sure this question is more suitable for Maven Central guys, but I am wondering if you can really rely on the IP address pool even if it exists. I am sure that the infrastructure will be very dynamic, as it serves libraries for the whole world!
I found this IP range for Maven Central while searching for the same. This can change very quickly without any notice or even be out of date.
You can also try the mirror center, which can be used through a single IP address and use it. Again, there is no guarantee that the IP address will never change.
Alternatively, you can configure the local Maven repository and place it in some kind of DMZ. Your network guys can either allow all machines from the DMZ or this particular machine (using IP) to access all outgoing HTTP, which in turn will allow him to access Maven Central. All your developers will use the internal repository as a mirror . This approach will also save your network bandwidth, as over time there will be fewer requests sent to the central part due to localization of local libraries.
I recon (3) is a better solution, although I do not know it directly.