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Difficulty: MediumIPv6 Addressing and Prefixes

Match each IPv6 addressing scenario or network description on the left with its corresponding prefix length or field bit length on the right.

  • The standard prefix length required for an enterprise end-user LAN subnet to support Stateless Address Autoconfiguration (SLAAC)/64
  • The typical prefix length allocated by a Regional Internet Registry (RIR) or ISP to an enterprise customer site/48
  • The RFC 6164 recommended prefix length for point-to-point inter-router link interfaces/127
  • The standard bit length allocated strictly for the host Interface Identifier (ID) portion of a Global Unicast Address64 bits

Answer

The SLAAC end-user LAN subnet requires a /64 prefix length; the enterprise customer ISP allocation is typically a /48 prefix length; the point-to-point inter-router link uses a /127 prefix length; and the host Interface Identifier occupies 64 bits.
Each scenario maps directly to IPv6 architectural conventions: SLAAC requires a /64 prefix, enterprise sites typically receive a /48 prefix, point-to-point inter-router links use /127 per RFC 6164, and the host Interface ID portion is 64 bits.

Step-by-Step Solution

1
Analyze SLAAC prefix length requirements.
SLAAC requires a /64 prefix length.
SLAAC relies on receiving a /64 prefix advertisement in ICMPv6 Router Advertisement (RA) messages to combine with a 64-bit Interface ID.
2
Determine standard enterprise IPv6 allocation from an ISP.
An enterprise customer site typically receives a /48 prefix.
A /48 global routing prefix leaves bits 49 through 64 (16 bits) available for the enterprise organization to construct up to 65,536 distinct /64 subnets.
3
Evaluate point-to-point link prefix standards.
Point-to-point router links use a /127 prefix length.
Per RFC 6164, using a /127 mask limits available addresses on the link to two, preventing ping-pong routing loops and ND CPU exhaustion attacks.
4
Identify the Interface ID length in a standard IPv6 unicast address.
The Interface ID is exactly 64 bits.
IPv6 address architecture divides a standard 128-bit address into a 64-bit network prefix and a 64-bit host Interface ID.

Key Concept

IPv6 Address Structure and Prefix Allocation Rules
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Topics

IPv6 Addressing and Prefixes

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