From RFC 5737
The blocks 192.0.2.0/24 (TEST-NET-1),
198.51.100.0/24 (TEST-NET-2), and 203.0.113.0/24 (TEST-NET-3) are provided for use in documentation.
From RFC 3849
IANA is to record the allocation of the IPv6 global unicast address
prefix 2001:DB8::/32 as a documentation-only prefix in the IPv6
address registry. No end party is to be assigned this address.