The IANA has allocated several special purpose domains and IP blocks for use as examples in code and documentation. Where possible, these domains and IP blocks should be preferred. There are some cases where it will not be possible to follow this guidance, for example, there is currently no reserved IDN domain name.
We prefer to use these names and IP blocks to avoid causing any unexpected collateral damage to the rightful owners of the non-reserved names and IP space. For example, publishing an email address in our codebase will more than likely be picked up by spammers, while published URLs etc using non-reserved names or IP space will likely trigger search indexers etc to begin crawling.
Reserved DNS domains are documented here: IANA Special Use Domain Names.
Several common reserved domains:
Reserved IP space is documented here: IANA IPv4 Special Registry, and IANA IPv6 Special Registry.
Several common reserved IP blocks: