Which is the correct fully compressed form of the IPv6 address 2001:0db8:0000:0000:0000:ff00:0042:8329?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals medium
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The options
Not correct 2001:db8:0:0:0:ff00:42:8329
Wrong only in that it is not fully compressed. Dropping leading zeros within each group is valid, but the run of three all-zero groups can additionally be collapsed to ::.
Correct 2001:db8::ff00:42:8329
Correct. Leading zeros in each group are dropped (0db8 -> db8, 0042 -> 42) and the single run of consecutive all-zero groups is replaced by ::.
Not correct 2001:db8::ff00::42:8329
Wrong, and invalid. :: may appear at most once in an address; two occurrences make the number of zero groups each one represents ambiguous, so parsers reject it.
Not correct 2001:db8:ff00:42:8329::
Wrong. This puts :: at the end, which claims the zero groups are the trailing ones. That is a different address entirely.
Why
IPv6 text compression has two rules: leading zeros inside a 16-bit group may be omitted, and exactly one run of consecutive all-zero groups may be replaced by ::. Because :: is position-sensitive, it must appear where the zeros actually were, and it may appear only once per address.
Where this comes from
- Cited
- LPI exam objective 109.1
- What it says
- Understand IPv6 address notation including zero compression.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 109 and space the ones you get wrong.