What does /48 mean in IPv6?

Answer

An IPv6 /48 leaves 16 bits for subnets before the /64 boundary, so it holds 65,536 /64 subnets. It is the traditional size for one site, such as an office, campus or data center.

Worked example: 2001:db8:abcd::/48. Type any block to compare.

2001:db8:abcd::/48 is 2^80 ≈ 1.2 septillion addresses: 2001:db8:abcd:: to 2001:db8:abcd:ffff:ffff:ffff:ffff:ffff. IPv6 has no broadcast address.

20010db8abcd00000000000000000000

48-bit prefix 80 bits for subnets and hosts (each hex digit is 4 bits)

Network: 2001:db8:abcd::
First address: 2001:db8:abcd::
Last address: 2001:db8:abcd:ffff:ffff:ffff:ffff:ffff
Addresses: 2^80 ≈ 1.2 septillion
Usable: ≈ 1.2 septillion
Expanded: 2001:0db8:abcd:0000:0000:0000:0000:0000
Range type: Documentation · not routed on the internet
/64 subnets inside: 65,536

Special-purpose range

Documentation 2001:db8::/32 RFC 3849
Reserved for examples in documentation. Never routed.

Split it into subnets

256 subnets, first 16 shown.

Subnet	Range	Addresses
`2001:db8:abcd::/56`	`2001:db8:abcd::1` – `2001:db8:abcd:ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:100::/56`	`2001:db8:abcd:100::1` – `2001:db8:abcd:1ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:200::/56`	`2001:db8:abcd:200::1` – `2001:db8:abcd:2ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:300::/56`	`2001:db8:abcd:300::1` – `2001:db8:abcd:3ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:400::/56`	`2001:db8:abcd:400::1` – `2001:db8:abcd:4ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:500::/56`	`2001:db8:abcd:500::1` – `2001:db8:abcd:5ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:600::/56`	`2001:db8:abcd:600::1` – `2001:db8:abcd:6ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:700::/56`	`2001:db8:abcd:700::1` – `2001:db8:abcd:7ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:800::/56`	`2001:db8:abcd:800::1` – `2001:db8:abcd:8ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:900::/56`	`2001:db8:abcd:900::1` – `2001:db8:abcd:9ff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:a00::/56`	`2001:db8:abcd:a00::1` – `2001:db8:abcd:aff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:b00::/56`	`2001:db8:abcd:b00::1` – `2001:db8:abcd:bff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:c00::/56`	`2001:db8:abcd:c00::1` – `2001:db8:abcd:cff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:d00::/56`	`2001:db8:abcd:d00::1` – `2001:db8:abcd:dff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:e00::/56`	`2001:db8:abcd:e00::1` – `2001:db8:abcd:eff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion
`2001:db8:abcd:f00::/56`	`2001:db8:abcd:f00::1` – `2001:db8:abcd:fff:ffff:ffff:ffff:ffff`	2^72 ≈ 4.7 sextillion

Is this IP in the range?

Range to CIDR blocks

What to know

The fourth group of the address becomes the subnet number: in 2001:db8:abcd::/48, the subnets run from 2001:db8:abcd:0::/64 to 2001:db8:abcd:ffff::/64. Many networks use that group to encode building, VLAN or environment.

A /48 is the minimum assignment an organization gets directly from ARIN, and RIPE-690 recommends a /48 for business customers of ISPs. RFC 6177 (2011) dropped the older advice that every end site, including homes, should get a /48; homes commonly get a /56.

On the public internet, /48 is the longest IPv6 prefix most networks accept in BGP, so a /48 is the smallest block you can usually announce yourself.

Gotchas

A /48 from your ISP belongs to that ISP. If you change providers you renumber; for multihoming or portability you need provider-independent space from a regional registry.
65,536 subnets is plenty, but use the bits deliberately. Changing a subnet numbering scheme later means renumbering every network.

Related explainers

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