IPv6 is short for "Internet Protocol Version 6". IPv6 is the "next generation" protocol designed by the IETF to ( 71 ) the current version Internet Protocol,IP Version 4("IPv4").
Most of today’s Internet uses IPv4, which is now more than twenty years old． IPv4 has been (72) resilient in spite of its age, but it is beginning to have problems. Most importantly, there is a growing ( 73 ) Of IPv4 addresses，which are needed by all new machines added to the Internet.IPv6 fixes a number of problems in IPv4，such as the (74) number of available IPv4 addresses. It also adds many improvements to IPv4 in areas such as routing and network autoconfiguration.IPv6 is expected to gradually replace IPv4，with the two coexisting for a number of years during a transition (75) .

A．substitution

B．swap

C．switchover

D．replace