IPv6 /48 prefix

A /48 leaves 80 bits for subnets and hosts: 1,208,925,819,614,629,174,706,176 addresses, or 65,536 /64 subnets.

IPv6 /48 at a glance

Mask: ffff:ffff:ffff::
Mask (expanded): ffff:ffff:ffff:0000:0000:0000:0000:0000
Host bits: 80
Addresses: 1,208,925,819,614,629,174,706,176
Nibble-aligned: Yes, the boundary falls on a hex digit
Reverse DNS zone: 0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

Typical uses of a /48

A /48 is the traditional end-site assignment (RFC 3177, relaxed by RFC 6177) and the longest prefix most networks accept in the global IPv6 BGP table, so it is the smallest block you can announce on the internet. A ULA prefix (fd00::/8, RFC 4193) is generated as a random /48, and Google Cloud gives each VPC a /48 ULA range.

Example: 2001:db8::/48

First address: 2001:db8::
Last address: 2001:db8:0:ffff:ffff:ffff:ffff:ffff
Last (expanded): 2001:0db8:0000:ffff:ffff:ffff:ffff:ffff
Next /48: 2001:db8:1::/48

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 /48 for an address

What a /48 contains

Smaller prefix	How many
/56	256
/60	4,096
/64	65,536
/112	18,446,744,073,709,551,616
/127	604,462,909,807,314,587,353,088
/128	1,208,925,819,614,629,174,706,176

How many /48s fit

In a	Number of /48s
/32	65,536

Cloud support for /48

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