IPv6 /127 Prefix ReferenceIPv6
IPv6's answer to the point-to-point question: a /127 carries exactly two addresses, one per router, and nothing else. RFC 6164 recommends it for inter-router links precisely because nobody else can even exist on the wire.
/127 facts at a glance
- Total addresses
- 2 (21)
- /64 subnets inside
- —
- Host/interface bits
- 1
- Example network
- 2001:db8::/127
- First address
- 2001:db8::
- Last address
- 2001:db8::1
- Typical role
- Point-to-point link (RFC 6164)
A /127 in documentation space
Using 2001:db8::/32 (RFC 3849 documentation space) aligned to its own boundary, a /127 occupies:
| Block | First address | Last address |
|---|---|---|
| 2001:db8::/127 | 2001:db8:: | 2001:db8::1 |
How an IPv6 /127 prefix works
A /127 prefix pins the first 127 bits of a 128-bit address, leaving 1 bits free. That free portion holds 21 = 2 distinct addresses, and every such block begins at a multiple of that size. Prefixes longer than /64 cannot use SLAAC; plan DHCPv6 or static assignment instead.
Where /127 is used
- Transit links between routers running OSPFv3, IS-IS, or eBGP.
- Hardened point-to-point circuits where neighbor discovery should never happen.
- Tidy address bookkeeping across hundreds of core links.
RFC 6164 endorses /127s for inter-router point-to-point links, closing the door on neighbor-discovery-based denial-of-service tricks that wider on-link subnets allow.
Think of it as IPv6's answer to RFC 3021: a /64 wastes nothing anyone cares about, but the /127 adds safety and cleaner operational habits.
Frequently asked questions
What is a /127 used for?
Point-to-point inter-router links. Each end takes one of the two addresses, and routing protocols form adjacency directly over the pair.
How many addresses does a /127 have?
Two, both usable on the link ends; there is no meaningful broadcast concept in IPv6 to reserve.
Why not just use a /64 for router links?
You could, but a /64 invites neighbor-discovery abuse from any compromised device claiming on-link status. The /127 removes that attack surface entirely.
Does SLAAC work on a /127?
No, and none is needed: router links get static or learned addresses, since there are no hosts present to self-configure.
Open this prefix in the calculator
Open 2001:db8::/127 in the IPv6 calculator