Monday, February 11, 2013

/96 prefix in ipv6/ipv4 mapping. The Why ?

Some one email me about the use of a /96 prefix in my ipv6v4 nat ( NAT 64 ) example. And here's the reason why, the  /96 prefix was created for the allowance of mapping  ipv4 32bit address into ipv6. If you think about it a /96 plus a /32 = 128 bits.

Here's the rfc  that explains some of this feature and mapping of ipv4 and ipv6 addres

ipv6/ipv4 mapping becomes more of an issues as we start dealing with DNS64 and ipv6 enabled networks using name-lookup  resources to look up AAAA (  A   ) records, but having the return query massage with the direct ipv4-to-ipv6 mapping.

When ever we use NAT64, we need to be aware of the differences between the two;


ttl           hop-limit
protocol field next-header
dscp/tos traffic  class
frag offsetfrag-header

When  ever we map ipv4 to ipv6, the 32 bit address is always mapped with the lower 32bit  address embedded

e.g  ( using ipv4 address and a 2001:179::/96 prefix ) =  A0:01 + /96 bit prefix    2001:179::10:0:0:1 or in hex format 2001:179::A0:01

 The above is  the proper way to map ipv4 to ipv6 address and using the /96 prefix

I hope this clears up  any confusion on way the /96 prefix.

Ken Felix
Freelance  Network & Security Engineer
kfelix at hyperfeed dot com

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