Plan around reserved values and the verified boundary
05Many router links can be numbered from one /64. The address-planning advice concerns the rightmost 64 bits, which is different from the single varying bit of a /127.
The illustrative lower64 values ::2 and ::3 avoid both listed planning concerns. Assign one to each router and disable Subnet-Router anycast for this /127; support is mandatory, universal deployment is not.
Source for this drawing: RFC 6164 · Read RFC section 6 ↗
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A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
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These pictures are analogies to help you learn. The linked RFC explains the actual rules.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The highest 128 values start at ffff:ffff:ffff:ff80, not ff7f. The SHOULD NOT recommendation and its address-planning context remain unchanged.
Read the updated RFC section ↗Assuming multiple point-to-point links are numbered from one /64, addresses with all zeros in the rightmost 64 bits SHOULD NOT be assigned as unicast, to avoid the Subnet-Router anycast collision.
- Assuming multiple point-to-point links are numbered from one /64, addresses with all zeros in the rightmost 64 bits SHOULD NOT be assigned as unicast, to avoid the Subnet-Router anycast collision.
- Under the same planning context, the highest 128 rightmost-64-bit values SHOULD NOT be used as unicast. Verified erratum 3422 corrects the start to ffff:ffff:ffff:ff80, through ffff:ffff:ffff:ffff inclusive.
- These are SHOULD NOT recommendations, distinct from MUST support /127 and MUST disable Subnet-Router anycast for its prefix. The listed checks are not a complete reserved-address, allocation, or routing validator.
- The chosen lower64 values ::2 and ::3 avoid both listed ranges. A zero last bit on ::2 is not the same as all 64 low bits being zero. Other reserved anycast formats, policies and address-management checks still matter.
Fictional documentation addresses within 2001:db8::/32. Bounded operator-chosen address examples, not a complete router, Neighbor Discovery, ICMPv6 or address-validation implementation.
Read the RFC section ↗