# IPv6 /127 Prefix – Point-to-Point Links (RFC 6164)

> An IPv6 /127 prefix contains 2 addresses. Mask math, examples, typical roles and FAQs for planning IPv6 networks.

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# 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.

An IPv6 /127 prefix contains 2 addresses. RFC 6164 recommends it for point-to-point router links.

## /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](https://itinsighthub.com/ipv6-subnet-calculator/?q=2001%3Adb8%3A%3A%2F127)

### Related IPv6 prefixes

[/29](https://itinsighthub.com/ipv6/29/)[/32](https://itinsighthub.com/ipv6/32/)[/34](https://itinsighthub.com/ipv6/34/)[/36](https://itinsighthub.com/ipv6/36/)[/40](https://itinsighthub.com/ipv6/40/)[/42](https://itinsighthub.com/ipv6/42/)[/44](https://itinsighthub.com/ipv6/44/)[/47](https://itinsighthub.com/ipv6/47/)[/48](https://itinsighthub.com/ipv6/48/)[/52](https://itinsighthub.com/ipv6/52/)[/56](https://itinsighthub.com/ipv6/56/)[/60](https://itinsighthub.com/ipv6/60/)[/64](https://itinsighthub.com/ipv6/64/)[/128](https://itinsighthub.com/ipv6/128/)

← IPv6 /64 prefix
IPv6 /128 prefix →

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