“The architectural intent of NAT is to divide the Internet into independent address administrations […] The result of this division is to enforce a client/server architecture (vs. peer/peer) where the servers need to exist in the public address realm.
A significant factor in the success of the Internet is the flexibility derived from a few basic tenets. Foremost is the End- to-End principle, which notes that certain functions can only be performed in the endpoints, thus they are in control of the communication, and the network should be a simple datagram service that moves bits between these points. Restated, the endpoint applications are often the only place capable of correctly managing the data stream. Removing this concern from the lower layer packet-forwarding devices streamlines the forwarding process, contributing to system-wide efficiency.
Another advantage is that the network does not maintain per connection state information. This allows fast rerouting around failures through alternate paths and to better scaling of the overall network. Lack of state also removes any requirement for the network nodes to notify each other as endpoint connections are formed or dropped. Furthermore, the endpoints are not, and need not be, aware of any network components other than the destination, first hop router(s), and an optional name resolution service. Packet integrity is preserved through the network, and transport checksums and any address-dependent security functions are valid end-to-end.
NAT devices (particularly the NAPT variety) undermine most of these basic advantages of the end-to-end model, reducing overall flexibility, while often increasing operational complexity and impeding diagnostic capabilities.”
From IETF RFC 2993: “Architectural Implications of NAT”, November 2000