NAT. The vast majority of end-users cannot accept incoming connections. The vast majority of end-users cannot initiate connections to each other. The only way

NAT. The vast majority of end-users cannot accept incoming connections. The vast majority of end-users cannot initiate connections to each other. The only way around that is centralization. BitTorrent users who are behind firewalls are only participating in a subset of the network - people who can receive incoming connections. That's okay because there's probably at least one who has the file you're looking for, but it wouldn't facilitate direct messaging. Skype was centralized on Supernodes. Two users make outbound connections to a Supernode, the Supernode passes traffic between the two parties. It was (sorta) decentralized because Supernodes were sometimes just random users who could accept incoming connections. Microsoft found it more reliable and scalable to just host the servers in datacenters instead of co-opting random PCs as servers. In our NATed, firewalled world, there is no such thing as true P2P between mainstream users.