IPv6 /127 prefix

A /127 leaves 1 bits for subnets and hosts: 2 addresses.

IPv6 /127 at a glance

Mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffe
Mask (expanded): ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffe
Host bits: 1
Addresses: 2
Nibble-aligned: No

Typical uses of a /127

A /127 has 2 addresses, one for each end of an inter-router link, as recommended by RFC 6164. It avoids the ping-pong problem that wider link prefixes can have. RFC 6164 says to disable the Subnet-Router anycast address on /127 links, because that address would otherwise be one of the two router addresses.

Example: 2001:db8::/127

First address: 2001:db8::
Last address: 2001:db8::1
Last (expanded): 2001:0db8:0000:0000:0000:0000:0000:0001
Next /127: 2001:db8::2/127

Uses the documentation prefix 2001:db8::/32 (RFC 3849). IPv6 has no broadcast address, so every address in the range is assignable, except where a platform reserves some.

Find the /127 for an address

What a /127 contains

Smaller prefix	How many
/128	2

How many /127s fit

In a	Number of /127s
/32	39,614,081,257,132,168,796,771,975,168
/48	604,462,909,807,314,587,353,088
/56	2,361,183,241,434,822,606,848
/64	9,223,372,036,854,775,808

Cloud support for /127

Platform	IPv6 subnet size	/127?
AWS VPC	/44 to /64 in steps of /4	No
Azure VNet	exactly /64	No
Google Cloud VPC	/64 (from the VPC's /48)	No