What are Unique Local Addresses (fc00::/7, e.g. fd00::/8) used for?

Unique Local Addresses are IPv6's "private" addresses — routed inside an organisation but never advertised to the global internet by border routers.

* Field layout of a unique local IPv6 address. — Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons. *

These are the IPv6 counterpart to IPv4's RFC 1918 private ranges (10.0.0.0/8, 192.168.0.0/16):

• Block: fc00::/7, with the fd00::/8 half used for locally-generated (randomised) prefixes
• Internal routers forward them between subnets in the company
• Border routers drop them — they never leak onto the public internet
• Good for internal services that should stay private regardless of which ISP prefix you currently have

Tip: Unique Local Addresses give you a stable internal addressing scheme even if your ISP-assigned global prefix changes — handy because IPv6 prefixes from providers are often not static.

Go deeper:

• Unique local address (Wikipedia) — fc00::/7, the randomised fd00::/8 global ID, and how ULAs differ from RFC 1918.
• RFC 4193 — Unique Local IPv6 Unicast Addresses — the defining spec, including the L-bit and random-prefix algorithm.