I'll set aside the obvious "why use Allegro" inquiry, and just deal with servers.
Well, since I have no idea what you need, then about all I can suggest is that which allows any C++ application to form TCP/IP connections over sockets.
Recent developments indicate that the Boost Asio library has been considered for acceptance into the C++ standard library, which suggests you consider it. I've used it long before it was ever considered for C++ standards and found it solid, reliable and efficient. With respect to game development, however, it is also quite "raw". That is, Asio focuses exclusively on making connections and sending/receive data. It can be used for both client and server development. In particular, when used to make server software it is robust and scalable.
What you do with the ability to send/receive data is then up to you, and that may be more of an issue for you dependent on your plans.