What are three advantages of IPv6 over IPv4?
The three most significant advantages to IPv6 over IPv4 are:
The increased address-space size to eliminate the risk of exhaustion.
More efficient routing via the use of fixed IP header sizes for the vast majority of traffic and more efficiently organized routing hierarchies.
Auto-configuration to reduce the effort of deploying network infrastructure and end hosts like workstations.
Other acceptable answers are: the elimination of the need for NAT, and the enablement of true P2P applications, though those are both closely tied to the increased address space size.
What is the biggest hurdle in the U.S. facing those planning an IPv6 transition?
The biggest hurdle is the installed base of IPv4 systems. Countries like China with no significant installed base are finding it much easier to deploy IPv6. Closely related to this hurdle is the dependence on the Internet and TCP/IP networks in general that the U.S. has for defense, finance, and other critical infrastructure.
Extra Credit: What was the dominant networking protocol prior to IP?
The Network Control Protocol (NCP) preceded TCP/IP NCP's shortcomings led to the development of TCP/IP, but the former survived on the Internet until the end of 1982 and longer in private networks, especially in the U.S. federal government. See
http://www.isoc.org/internet/history/brief.shtml for a brief history of the Internet by those ...