IPv6 /126 prefix

A /126 leaves 2 bits for subnets and hosts: 4 addresses.

IPv6 /126 at a glance

Mask: ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffc
Mask (expanded): ffff:ffff:ffff:ffff:ffff:ffff:ffff:fffc
Host bits: 2
Addresses: 4
Nibble-aligned: No

Typical uses of a /126

A /126 has 4 addresses and mirrors the IPv4 /30 habit for point-to-point links. RFC 6164 recommends /127 instead, but some older equipment and operator conventions still use /126.

Example: 2001:db8::/126

First address: 2001:db8::
Last address: 2001:db8::3
Last (expanded): 2001:0db8:0000:0000:0000:0000:0000:0003
Next /126: 2001:db8::4/126

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 /126 for an address

What a /126 contains

Smaller prefix	How many
/127	2
/128	4

How many /126s fit

In a	Number of /126s
/32	19,807,040,628,566,084,398,385,987,584
/48	302,231,454,903,657,293,676,544
/56	1,180,591,620,717,411,303,424
/64	4,611,686,018,427,387,904

Cloud support for /126

Platform	IPv6 subnet size	/126?
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