If you're writing documentation for stuff that's used on the Internet, why not use the domain names and number resources that have been set aside for that purpose? Here they all are in one easy set, with references.
Soapbox: Although it's a fairly common practice I'm opposed to the use of IPv4 private (RFC1918) space or IPv6 ULA space for documentation, unless the thing that is being documented is a scenario in which said private space would be deployed in real life. For documentation of public-facing resources, use of appropriate documentation-reserved name and number resources will help avoid confusion.
(I'll cop to filing at least one patent application that included real live IPv6 space as examples rather than the documentation prefixes. Bad RS)
Domain names for documentation (described in RFC 2606):
IPv4 blocks for documentation (described in RFC 5737):
IPv6 block for documentation (described in RFC 3849):
Autonomous System Numbers for documentation (described in RFC 5938):
- (16-bit): 64496 - 64511
- (32-bit): 65536 - 65551