3fff::/20

3fff

Last updated 2 Nov 2026 from the IANA IPv6 Special-Purpose Address Registry

Short answer

3fff::/20 is the larger IPv6 documentation prefix, for examples that need more room than 2001:db8::/32.

Block	3fff::/20
First – last	3fff:: – 3fff:fff:ffff:ffff:ffff:ffff:ffff:ffff
Size	a /20 (2^108 addresses)
Name in the registry	Documentation
Defined in	RFC 9637
Allocated	2024-07
Valid as source / destination	False / False
Forwardable	False
Globally reachable	False

What it means

RFC 9637 added 3fff::/20 in 2024 so that documentation can show realistic multi-site IPv6 plans. Like 2001:db8::/32, it is never routed.

Seeing it outside a manual means a configuration was copied from an example.

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