When using zero-compression on the following IPv6 address
2001:0DB8:0000:CD30:0000:0000:0000:0000/60 Why is this not correct:
2001:DB8::CD30::/60 ... while this is:
2001:DB8:0:CD30::/60 
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3 Answers
Zero compression can only be made once. The reason for this is, that the IPv6 address is not unique any more otherwise.
Take your example 2001:DB8::CD30::/60 will it expand to
2001:0DB8:0000:0000:0000:CD30:0000:0000/60 or
2001:0DB8:0000:0000:CD30:0000:0000:0000/60 or
2001:0DB8:0000:CD30:0000:0000:0000:0000/60 ...?
If only one "::" is used, the result will always be unique as there is only one possible fixed number of zeros to be inserted.
Comments
Because it is ambiguous.
The address 2001:DB8::CD30:: could be expanded in any of the following possibilities:
2001:DB8:0:CD30:0:0:0:0
2001:DB8:0:0:CD30:0:0:0
2001:DB8:0:0:0:CD30:0:0
2001:DB8:0:0:0:0:CD30:0
1 Comment
Jeremy Visser
Why the downvote? This answer is objectively correct.
The reason is that :: is used to shorten multiple zeros in the 16-bit address field.
In your example 2001:0DB8:0000:CD30:0000:0000:0000:0000/60, it only has multiple 0s in the 16-bit field at the suffix, the 0000 in 2001:0DB8:0000:CD30: is just one 16-bit field and you'd just use 0 to shorten it.
More interesting question: How would you shorten this2001:0000:0000:CD30:0000:0000:0000:0000/60?
It is defined in the standard:
In addition, Section 2.2 of [RFC4291] notes,
'The "::" can only appear once in an address.'
What it means that the address can be written as either: 2001:0:0:CD30::/60 OR 2001::0:CD30:0:0:0:0/60.
Both are valid, but I'd prefer the first representation since the purpose of zeroco mpression is to shorten the address where the first representation is shorter.
