Why 128 bit ? - It was a compromise between a fixed 64 bit and a variable 160 bits (google OSI NSAP for more)
How many IP addresses ? - 340,282,366,920,938,463,463,374,607,431,768,211,456
What about IPv5 ? - Used by ST - a stream protocol and not related to IP
Key Differences
- More IPs
- Different Header format
- Extension support
- Flow labeling
- Authentication support
Header
- Very simplified (See http://en.wikipedia.org/wiki/File:Ipv6_header.svg)
- 20 bytes + Options in IPv4 Vs 12 fields (40 bytes) in IPv6
- Base header no longer contains fragmentation options
- Header no longer contains any checksum
- Time to live (TTL) is now called Hop Limit
- Support for traffic classes
- Extensibility in headers. Options are not limited to the 40 bytes
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