From https://tools.ietf.org/html/rfc2606
 
The Internet Assigned Numbers Authority (IANA) also currently has the following second level domain names reserved which can be used as examples.
        example.com
        example.net
        example.org
 
From https://tools.ietf.org/html/rfc5737
 
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 https://tools.ietf.org/html/rfc3849
 
The prefix allocated for documentation purposes is 2001:DB8::/32