What is IPv6?

The answer to IPv4 address space problem
IP address expanded from 32bit to 128bit
4x10^9 -> 3.4x10^38
4 billion -> 340 undecillion (thanks to /usr/games/number)

A new starting point
IPv4: 20+ years, minimal implementation lacks almost everything
IPv6: new standard features: multicast, PMTU, IPsec, autoconfig
Improve availability of advanced technologies

Say a long good-bye to NAT, say hi again to end-to-end model
NAT breaks bidirectional communication
NAT is a bondage to application protocol designers
NAT does not let multicast/IPsec through
NAT does not work with proprietary protocols
NAT does not improve security
NAT box is a single point of failure