IPv4 address space exhaustion. NAT breaks end-to-end connectivity, making entire classes of applications difficult to deploy, and this is made worse when the N

IPv4 address space exhaustion. NAT breaks end-to-end connectivity, making entire classes of applications difficult to deploy, and this is made worse when the NAT gateway is not under a user's control. NAT gateway deployed by ISPs represent a violation of the end-to-end principle and have to be even more complex and stateful than the NAT gateways users often deploy. If you are looking for a specific problem NAT introduces, one good one is that NAT forces users and application developers to use protocols that the NAT gateway properly supports (e.g. TCP, UDP) and to only use those protocols in ways that are "NAT-friendly" (e.g. easy-to-track client-server sessions that are initiated from the private side the gateway) or else to use some NAT-specific protocol as a crutch (UPnP/IGD, STUN, etc.). When both ends of a two-party protocol are behind NAT gateways it can become even more painful and possibly require a third party to be introduced to an application that is otherwise unnecessary for the application. So at a minimum IPv6 gets us back to the end-to-end principle where Internet peers connect directly to each other without having to negotiate with the network itself. There are a few other advantages, like the fact that users can get large, publicly-routed addresses spaces for their own use (e.g. ISPs giving users a /56 prefix), simplified bridging of private networks (very low probability of collisions in the private IPv6 range), simplified router configuration (link-local addresses are always available so there is no need to assign numbers to every link), and other assorted niche benefits. In theory IPv6 should mean routers become more efficient because it is easier to aggregate prefixes, which should generally benefit Internet users by reducing latency (though at this point there is not much room left for improvement there).