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Difficulty: Very hardIPv6 Addressing Concepts and Header Structure

A network engineer is analyzing IPv6 header fields and address representation standard rules for an enterprise core migration. Which of the following statements correctly describe IPv6 header functionality and address compression standards? (Select TWO)

  1. The 20-bit Flow Label field enables routers to identify and maintain consistent forwarding paths for specific packet flows without inspecting extension headers or upper-layer payload headers.Answer
  2. Applying double colons (::) multiple times within a single IPv6 address is prohibited because it prevents deterministic reconstruction of the 128-bit address.Answer
  3. C
    The address 2001:0db8:0000:0000:0005:0000:0000:0001 can be validly shortened to 2001:db8::5::1 by placing double colons around isolated non-zero fields.
  4. D
    The 8-bit Hop Limit field in the main IPv6 header specifies the maximum Quality of Service (QoS) priority assigned to a packet.

Answer

The correct statements are that the 20-bit Flow Label field enables routers to maintain consistent flow handling without inspecting extension headers, and that multiple double colons (::) are prohibited in IPv6 address compression because they cause ambiguous expansion.
The 20-bit Flow Label header field allows routers to maintain consistent path forwarding for specific packet streams without parsing extension headers. In addition, IPv6 address compression rules strictly forbid using the double-colon sequence more than once in an address representation to ensure the 128-bit total length can be unambiguously reconstructed.

Step-by-Step Solution

1
Evaluate the role of the Flow Label field in the IPv6 header structure.
Confirm that the 20-bit Flow Label field identifies specific non-default traffic flows for QoS and routing without needing deep packet inspection.
Flow Label allows intermediate nodes to perform flow-based routing efficiently.
2
Analyze RFC 5952 IPv6 address compression syntax rules.
Verify that double-colon (::) zero-compression can strictly be used only once per address string.
If used twice, the number of zeros replaced by each double-colon is indeterminate.
3
Inspect the remaining distractor options regarding multiple double colons and the Hop Limit field.
Identify that 2001:db8::5::1 is invalid syntax and that Hop Limit acts as a decrementing hop counter for loop prevention rather than a QoS priority indicator.
Hop Limit replaces IPv4 TTL, while Traffic Class defines IPv6 QoS parameters.

Key Concept

IPv6 Header Fields and RFC 5952 Compression Rules
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Topics

IPv6 Addressing Concepts and Header Structure

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