With IPsec, the protocols for IPv4 and IPv6 are fairly similar, and for the most part, there is a single set of standards that cover both IPv4 and IPv6. The main difference is the almost universal presence of NAT in IPv4 and total absence of it in IPv6. With Mobile IP, on the other hand, the support for IPv4 and IPv6 are quite different, each has its own set of standards, and they continue to diverge more and more over time. Mobile IPv6 is much more capable than Mobile IPv4, and due to the imminent exhaustion of the IPv4 address space, most new research is being done on Mobile IPv6, not Mobile IPv4. Again, the biggest problem with Mobile IPv4 is the presence of NAT in most real-world IPv4 networks.

SOURCE: The Second Internet, book authored by InfoWeapons Chairman and CTO Lawrence E. Hughes.