At that time a 32 bit address field seemed sufficient to allow millions of addresses. The exponential growth of the Internet now seems to on the verge of consuming all available addresses. There can be a variable number of extra headers Extensible protocol additional features can easily be added. No checksum in the base header to allow for faster processing. A new base header and fragmentation header is added to each fragment.
Source is required to fragment.
Routers no longer fragment packets. Slow adoption Networks are expected to support both IPv4 and IPv6. Inter IP type packets can be converted or tunneled.