fc00::/7: unique local addresses

Answer

fc00::/7 is reserved for unique local addresses (ULA, RFC 4193), IPv6's counterpart to the private IPv4 ranges. Only the fd00::/8 half is used: you pick a random 40-bit global ID, which gives your site its own /48 such as fd3c:8a91:52e0::/48.

fc00::/7 is 2^121 ≈ 2.7 undecillion addresses: fc00:: to fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff. IPv6 has no broadcast address.

fc000000000000000000000000000000

7-bit prefix 121 bits for subnets and hosts (each hex digit is 4 bits)

Network: fc00::
First address: fc00::
Last address: fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff
Addresses: 2^121 ≈ 2.7 undecillion
Usable: ≈ 2.7 undecillion
Expanded: fc00:0000:0000:0000:0000:0000:0000:0000
Range type: Unique local address (ULA) · not routed on the internet
/64 subnets inside: 2^57 ≈ 144 quadrillion

Special-purpose range

Unique local address (ULA) fc00::/7 RFC 4193
IPv6's counterpart to private ranges. In practice fd00::/8 with a random 40-bit ID.

Split it into subnets

256 subnets, first 16 shown.

Subnet	Range	Addresses
`fc00::/15`	`fc00::1` – `fc01:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc02::/15`	`fc02::1` – `fc03:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc04::/15`	`fc04::1` – `fc05:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc06::/15`	`fc06::1` – `fc07:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc08::/15`	`fc08::1` – `fc09:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc0a::/15`	`fc0a::1` – `fc0b:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc0c::/15`	`fc0c::1` – `fc0d:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc0e::/15`	`fc0e::1` – `fc0f:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc10::/15`	`fc10::1` – `fc11:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc12::/15`	`fc12::1` – `fc13:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc14::/15`	`fc14::1` – `fc15:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc16::/15`	`fc16::1` – `fc17:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc18::/15`	`fc18::1` – `fc19:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc1a::/15`	`fc1a::1` – `fc1b:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc1c::/15`	`fc1c::1` – `fc1d:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion
`fc1e::/15`	`fc1e::1` – `fc1f:ffff:ffff:ffff:ffff:ffff:ffff:ffff`	2^113 ≈ 10.4 decillion

Is this IP in the range?

Range to CIDR blocks

What to know

The random ID is the point. Two organizations that each picked a random ULA /48 are very unlikely to collide, so merging networks or connecting them by VPN rarely requires renumbering, unlike two networks that both chose 10.0.0.0/8.

ULA is useful for internal addresses that must stay the same when the ISP-assigned prefix changes, such as DNS servers, printers or management interfaces. Hosts can have a ULA and a global address at the same time.

Gotchas

The fc00::/8 half was meant for centrally assigned ULAs that were never defined. Do not use it; use fd00::/8.
Do not use fd00::/48 or other easy-to-type prefixes. That defeats the random ID and brings back the collision problem.
ULA is not routed on the internet, and many systems prefer IPv4 over ULA when choosing a source address. Do not expect ULA-only hosts to use IPv6 for external traffic.

Related explainers

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