There are a few older games (Quake III and Hexen II to name a couple) where the server assigns a port per player.
Are there performance advantages to this? I suspect there may be, since Carmack & Co.'s endeavours tend to lean in that direction.
I found this, which doesn't provide many different points of view... unfortunately the Book of Hook (link therein) has been down of late, and I don't know if it will ever become available again. What a waste of valuable oldschool knowledge. (If anyone knows where else this might be available, I would be indebted.)