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So I am new to the IPv6 concept, and I seen that the Protocol field seems to be replaced with Next Header, and I have a question.

Question:

Is Next Header a complete replace of Protocol, or is they are actually some minor differences? If so what are their differences?

Based on some of my reseach , some say it v4 Protocol is similar to v6 Next Header 1, some say it is the same 2.

My thought is there must be some kind of difference, otherwise why they will brother to completely change the name of the field. Thanks.

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    Basically you are right, is the same thing, the only thing is that may be the name "next header" leaves open the door so you can think that may be there is more "next headers", this concept is more spread on IPv6 than IPv4 but in essence are the same thing – camp0 Dec 3 '19 at 10:17
  • It can be the same, and the very last Next Header field is the same, but it can also be very different. – Ron Maupin Dec 3 '19 at 14:11
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IPv4 didn't have many extensibility options. The header had an optional options field and that was it. So every packet consisted of the IPv4 header (possibly with options) and the protocol like TCP, UDP, ICMP, SCTP etc.

IPv6 was made a lot more extensible. They kept the basic header much simpler and moved everything that wasn't strictly necessary for every packet into extension headers. So a packet can contain for example the IPv6 header, a fragmentation header and TCP/UDP/etc. New developments like SRm6 make use of a CRH routing header and the destination options header.

So in IPv6 there is a chain of headers, and while the next header might well be a TCP or UDP protocol header, it might also be an extension header. Therefore the new name is appropriate.

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