Why use a /127 for point-to-point links?

RFC 6164 recommends /127 for links between two routers. With only two addresses there is no room for a neighbor-discovery exhaustion attack, and no addresses are wasted on a link that only ever has two endpoints.

What is the difference between /127 and /128?

A /127 is twice as large as /128. /127 has 2¹ addresses; /128 has 2⁰. Each step down the prefix number doubles the address count.

How does /127 compare to /126?

A /126 is twice as large as /127. Moving from /127 to /126 adds one more fixed network bit, halving the host portion from 2¹ to 2² addresses.

How do I calculate an IPv6 /127 subnet?

Enter an IPv6 address and select prefix /127 in the calculator above — or paste full CIDR notation like 2001:db8:abcd:0012::1/127. The tool instantly returns the network address, address range, prefix mask, reverse DNS, and total count of 2¹ addresses.

IPv6 /127 Subnet Calculator

Results update as you type

A /127 provides exactly 2 addresses and is the recommended prefix for point-to-point links between routers (RFC 6164). Using a /127 avoids the neighbor-discovery cache exhaustion issues that can affect larger link subnets.

Prefix length

/128

/127 = 2¹ addresses (≈ 2)

Results for 2001:db8:abcd:12::/127

Documentation (RFC 3849)Global scope

Network / prefixThe first address — identifies the subnet itself

First addressSubnet-router anycast; first address in the block

Last addressThe highest address in this block

Prefix maskEquivalent to /127

Total addresses≈ 2 addresses

Address typeGlobally routable scope

More detailsNeighboring subnets, expanded address, reverse DNS, and hextet breakdown

Neighboring /127 subnets

Expanded address
Compressed address
Network (expanded)
Last address (expanded)
Prefix mask
Total addresses (exact)
Reverse DNS (PTR)
Host bits / network bits

Hextet breakdown

20010db8abcd00120000000000000001

NetworkSplit groupHost

Quick facts for IPv6 /127

Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffe
Total addresses: 2¹
Approx. count: 2
/64 subnets: —

IPv6 /127 reference guideBit split, overview, key facts, sizing tables, design notes, standards, and FAQ

Network / host bit split

netnetnetnetnetnetnetmix

Network bits (127)Split hextetHost bits (1)

Network bits: 127
Host bits: 1
Prefix mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffe
Total addresses: 2¹
Approx. count: 2
/64 subnets: —
Addresses formula: 2^1
/64 relationship: smaller than a /64

Overview

Common use cases

Router-to-router point-to-point links (RFC 6164)
Backbone and transit interconnects

Key facts

A /127 fixes 127 network bits and leaves 1 host bits — 2¹ total addresses.
In network design terms, /127 is typically a recommended point-to-point link.
Written out, /127 holds exactly 2 addresses.
A /127 is 1/2⁶³ of a standard /64 LAN subnet.

Design guidance

Use /127 for point-to-point links between two routers. Assign one address to each end — no need for a network or broadcast address. Modern IOS, Junos, and Linux all support /127 on transit links. Avoid using /127 on multi-access segments like Ethernet LANs.

Practical example

On a WAN link, router A might use 2001:db8::0/127 and router B uses 2001:db8::1/127 — exactly two addresses, one per endpoint, with no wasted space.

Related RFCs and standards

RFC 6164Using 127-Bit IPv6 Prefixes on Inter-Router Links

Prefix sizing reference

Divide /127 into…

Prefix	Subnets	Addresses each
/128	2	1

/127 fits inside…

Supernet	Addresses	/127s inside
/126	2²	2
/125	2³	4
/123	2⁵	16
/119	2⁹	256

Frequently asked questions

Common questions about IPv6 /127 blocks, prefix sizes, and use cases.

Common IPv6 prefix sizes

/3 /12 /16 /20 /24 /28 /32 /36 /40 /44 /48 /52 /56 /60 /64 /80 /96 /112 /120 /124 /126 /127 /128