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Difficulty: HardIPv6 Addressing Concepts and Header Structure

A network administrator is evaluating IPv6 header structures and address formatting rules prior to deploying Quality of Service (QoS) and traffic filtering policies on an enterprise router. Which of the following statements correctly describe IPv6 header field operations or standard address notation rules? (Select TWO.)

  1. The Next Header field specifies the protocol type of the header immediately following the main IPv6 header, functioning similarly to the IPv4 Protocol field while also enabling extension header chaining.Answer
  2. Standard IPv6 address canonical representation under RFC 5952 requires using lowercase hexadecimal characters and compressing the single longest sequence of consecutive 16-bit zero fields with a double colon (::).Answer
  3. C
    An IPv6 address shorthand notation containing multiple double colons, such as 2001:db8::1::56, is valid as long as the total expanded address length equals 128 bits.
  4. D
    The 20-bit Flow Label field in the IPv6 header is decremented by each intermediate Layer 3 router to prevent packets from endlessly looping across the network.

Answer

The Next Header field indicates the payload protocol or subsequent extension header, and RFC 5952 standardizes address notation by requiring lowercase hexadecimal representation and allowing only a single double-colon compression for the longest consecutive zero fields.
The IPv6 Next Header field serves a dual purpose by either specifying an extension header in a chained sequence or identifying the transport layer protocol payload (such as TCP or UDP). Additionally, RFC 5952 standardizes IPv6 address formatting by mandating lowercase hexadecimal characters and permitting only one double-colon (::) compression for the single longest contiguous run of zero hextets.

Step-by-Step Solution

1
Analyze the role and mechanics of the IPv6 Next Header field.
The 8-bit Next Header field in the IPv6 header identifies the protocol type of the header immediately following it. This can be an IPv6 Extension Header (e.g., Hop-by-Hop Options, Routing, IPSec ESP) or an upper-layer protocol (e.g., TCP value 6, UDP value 17), providing flexible protocol chaining.
Understanding header structure ensures proper filter placement and protocol inspection.
2
Evaluate IPv6 address compression rules per RFC 5952.
RFC 5952 establishes canonical formatting rules for IPv6 addresses: leading zeros within a 16-bit block must be suppressed, hexadecimal digits must be lowercase, and the double colon (::) can only be used once across the entire address to compress the longest run of consecutive 16-bit zero blocks.
Adhering to canonical address compression rules avoids syntax errors and ambiguity.
3
Identify misconceptions in wrong statements.
Using double colons twice (::) in a single address creates ambiguity because it is mathematically impossible to resolve how many zero blocks each '::' represents. Furthermore, the Hop Limit field is responsible for loop prevention by decrementing at each hop, while the Flow Label field identifies specific non-default traffic flows for real-time processing.
Differentiating header fields and compression constraints eliminates incorrect choices.

Key Concept

IPv6 Header Structure and Address Representation
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Topics

IPv6 Addressing Concepts and Header Structure

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