RFC 6164 / START HERE
IPv6 /127 router links, before the details
IPv6 means Internet Protocol version 6; /127 says that 127 of an address’s 128 bits identify the prefix.
This RFC explains how to assign a two-address IPv6 prefix to a point-to-point link with exactly two routers and no hosts.
The idea in one picture
Check the two-router scope
Assign the two endpoints
Apply anycast and planning rules
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
A small, fully assigned pair avoids particular unused-address forwarding and neighbor-cache problems. The routers must support /127 and disable Subnet-Router anycast for that prefix; it is not a requirement to change every IPv6 link.
A familiar example
Think of a private corridor with two numbered doors: there are no extra unused door numbers inside that pair. The real rules apply to configured inter-router links, exclude link-local scope, and do not prevent every routing loop or denial-of-service attack.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Choose a two-router link and a two-address prefixIPv6 means Internet Protocol version 6. Its addresses have 128 bits. A /127 fixes 127 bits and leaves one bit, giving two addresses for a deliberately configured point-to-point router link.→
- Disable Subnet-Router anycast for this prefixAnycast delivers to one member of a set. The Subnet-Router anycast role is distinct from an ordinary interface address; its historical conflict explains an essential /127 requirement.→
- Stop a datagram from bouncing back on its linkSome point-to-point media do not use Neighbor Discovery. A larger prefix can leave unused destinations that neither router owns, creating a ping-pong risk if the required forwarding check is missing.→
- Remove unused on-link targets from the cache problemNeighbor Discovery, or ND, resolves an IPv6 neighbor to a link-layer address. On a large on-link prefix, many unassigned targets can cause unresolved neighbor entries and repeated solicitation work.→
- Plan around reserved values and the verified boundaryMany 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.→
This introduction explains RFC 6164 (2011). Read the original RFC ↗