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Difficulty: MediumIPv6 Addressing Concepts and Header Structure

A network engineer attempts to configure a static route on a enterprise router using the destination IPv6 address 2001:db8:1000:1::0:100::5. Upon executing the command, the router CLI rejects the entry and displays a syntax error. Which of the following best explains why this IPv6 address representation is invalid?

  1. The address uses the double-colon shorthand (::) more than once, creating ambiguity in the total number of zero-padded 16-bit hextets.Answer
  2. B
    The address omits leading zeros within the hextet blocks, which violates RFC 5952 IPv6 compression rules.
  3. C
    The double-colon shorthand (::) is restricted to the start or end of an IPv6 address string and cannot appear in middle blocks.
  4. D
    The hexadecimal characters in the address are written in lowercase letters rather than required uppercase letters.

Answer

The address uses the double-colon shorthand (::) more than once, creating ambiguity in the total number of zero-padded 16-bit hextets.
The IPv6 specification mandates that the double-colon shorthand (::) can only be used ONCE in any single address string. Because an IPv6 address must expand to exactly eight 16-bit hextets (128 bits total), having two instances of :: makes it impossible for the system parser to determine how many zero blocks each :: represents.

Step-by-Step Solution

1
Analyze the full structural components of an IPv6 address.
An IPv6 address consists of 128 bits divided into eight 16-bit blocks (hextets) separated by colons.
Understanding the 8-hextet baseline allows accurate expansion of compressed IPv6 representations.
2
Evaluate the provided IPv6 string 2001:db8:1000:1::0:100::5 for shorthand compression compliance.
The string contains two separate instances of the double-colon (::) operator.
The double-colon represents one or more contiguous hextets of all zeros.
3
Determine the impact of using multiple double-colon operators.
When expanding two double colons, the exact count of zero hextets represented by each double colon cannot be unambiguously calculated.
Because of this mathematical ambiguity, networking software and CLI parsers reject addresses containing more than one double colon as invalid syntax.

Key Concept

IPv6 Address Compression Rules (RFC 5952)
Estimated Time:1m 0s
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Topics

IPv6 Addressing Concepts and Header Structure

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