I have started an educational project to learn about networking. I built a small server using enet that receives UDP packets and replies to clients, next step is to actually have multiple connected clients. I dont plan to create my own WoW killer mmorpg, but I would like to learn the right way to do things. I was plannnig to use an std::map to eep the list of clients indexed by their ip address, but then I thought that it wouldnt be optimal when the server needs to find client neighbors to propagate changes (I think that kdtrees are the right data structure to optimize that). Then I thought that I should have an std::map indexed by IP and a kdtree with "visible" entities. And finally, I realized that using IPs mnaybe is not a wise way to index clients, because it would limit connections to one per IP. What is the correct approach to this problem?

Typically, you keep several data structures with "clients" or "client entities" in them.
You'll want a hash table (typically unordered_map, not regular map) from source IP:port to client.
You'll also want a spatial index, as you say a kd tree or perhaps easier a loose quadtree.
You may also want various queues for notifications, periodic tasks, database checkpoints, etc.