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Question

Difficulty: EasyIPv6 Addressing Concepts and Header Structure

A network administrator attempts to configure a static route using the IPv6 address representation `2001:db8::100::1`. The configuration tool rejects the input as syntactically invalid. Which of the following explains why this IPv6 address representation is invalid?

  1. The double colon symbol is used more than once in a single address, introducing ambiguity regarding the number of missing zero blocks.Answer
  2. B
    The double colon shorthand can only compress consecutive zeros if they appear at the beginning or end of an address.
  3. C
    Leading zeros within a 16-bit hexadecimal block cannot be omitted when using double colon compression.
  4. D
    The double colon shorthand is reserved exclusively for link-local addresses and cannot be used in global unicast addresses.

Answer

The double colon symbol is used more than once in a single address, introducing ambiguity regarding the number of missing zero blocks.
In IPv6 text representation, the double colon (`::`) replaces one or more contiguous 16-bit blocks of zeros. Because a complete IPv6 address must contain exactly 128 bits (eight 16-bit hextets), placing `::` twice in a single address creates ambiguity as to how many zero blocks each double colon is intended to expand into.

Step-by-Step Solution

1
Analyze the given IPv6 address structure
The address string `2001:db8::100::1` contains two instances of the double colon (`::`) shorthand operator.
An uncompressed IPv6 address consists of 128 bits divided into eight 16-bit hexadecimal blocks separated by colons.
2
Apply standard IPv6 address shorthand rules
The double colon (`::`) represents contiguous blocks of 16-bit zero fields, but may only appear ONCE in an address string.
If used multiple times (e.g., `2001:db8::100::1`), it is mathematically impossible to resolve whether the first `::` replaces 3 zero blocks and the second replaces 2, or vice versa, making the full 128-bit address ambiguous.

Key Concept

IPv6 Address Compression Rules (RFC 5952)
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Topics

IPv6 Addressing Concepts and Header Structure

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