Actually not germane to the original question, but you can have multiple processes listening on the same port. Perhaps the most familiar instance of this would be apache where each of the child httpds can have a copy of the same socket descriptor (and use a locking mechanism on some OSen (older Solaris in particular) where multiple processes calling accept isn't handled correctly). Hypothetically you could have intelligence enough in each server process to determine what protocol was being spoken and do a file descriptor send (aka deep POSIX IPC wizardry) to another process if the one that received the connection can't handle it, but I can't think of any practical benefit to doing so.