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Private domain names

I'm positive the IP prefixes reserved for private use by RFC 1918 are well-known to anyone building private IP networks. Likewise, you should be familiar with reserved AS numbers documented in RFC 1930 if you're building private networks running BGP. But if you know there are reserved DNS domains that can be used to write sample configurations and test code, you're smarter than I was a few weeks ago.

I was writing the June IP Corner article and needed to set up DNS servers within the lab. I used as the domain name and decided to check what would happen if you'd visit the actual web site (try it out). It politely referenced me to RFC 2606, which documents the reserved domain names you can use.

As a rule, you should use private IP addresses, AS numbers and domain names in all technical documentation you're producing (unless, of course, you're describing an actual network). If you're forced to use public addresses or AS numbers (for example, to illustrate how the neighbor remote-private-as command works), you should clearly state that the AS numbers are imaginary.


  1. You may be interested to know that there is also a reserved IP range for testing as well documeneted in RFC2544. I blogged it here

    (Hope its OK to reference the article)

  2. For a laugh, have a read of the Spam Filtering writeup on -

    This guy has been suffering from non-compliance with RFC 2606 for years, his domain even appeared in the 1999 HTML 4.01 spec as an example domain - and he has had to really look into blocking spam.
  3. @gregferro: Of course it's OK to reference the article relevant to the topic (and I would silently delete the comments about blue life-enhancing pills :).
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