fc00::/7 (IPv6 Unique Local (ULA))
PrivateRFC 4193
fc00::/7 at a glance
| CIDR block | fc00::/7 |
|---|---|
| First address | fc00:: |
| Last address | fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff |
| Total addresses | 2¹²¹ |
| Prefix mask | fe00:: |
| Prefix length | /7 |
| Type | Private |
| Publicly routable | No |
| Defined in | RFC 4193 |
What is fc00::/7 used for?
fc00::/7 is the IPv6 unique local address (ULA) range from RFC 4193, the IPv6 counterpart of private IPv4 ranges. In practice only fd00::/8 is used: each site picks a random 40-bit Global ID to form its own /48. ULA addresses are not routed on the internet. Tailscale assigns device IPv6 addresses from fd7a:115c:a1e0::/48.
FAQ
What is the difference between fc00::/7 and fd00::/8?
fc00::/7 is the whole ULA block. Only its fd00::/8 half is defined for use, with a random 40-bit Global ID; fc00::/8 is reserved for a centrally assigned scheme that was never created.
Is fd00:: the IPv6 equivalent of 192.168.x.x?
Roughly. ULA addresses are not routed on the internet and are used internally, but IPv6 networks usually also have global addresses, so NAT is not normally needed.