You want to make a presentation, write a blog post or document your brand-new software. What do you use as domain-names, IP addresses and AS numbers? If you are documenting your systems or a setup you build for a customer the question is easy to answer: It’s best to use the actual data. But if you are writing a more generic documentation there are some reserved domain names, IP prefixes and AS numbers.

Why?#

But first let’s ask why we should use specialized values and not some random stuff you made just up.

People like to copy stuff from documentation. At least a couple of years ago “cisco” was the most common password on any Cisco router or Switch. No it was not a default password. But it was the password that was in every documentation. Either from on the website or any of the training materials¹

If you believe rumors a big German government agency is another example for that. They use the IPv4 address range from a university. Not because they randomly picked a prefix, but because before they implemented IPv4 (and yes there was a time before IP!) they had training and consulting and the training materials where prepared by a student form that university. So they took the prefix they knew from the documentation and used it for their IP addressing plan.

It is also bad idea to just make up domain names. Years ago there was a very controversial article in a magazine with a Domain name as a title. A friend noticed that the domain wasn’t reserved, registered it and published an article describing what was wrong with the article in the magazine.

Some time ago I also saw some advertising showing a domain-name which was pointing to the website of a competitor.

What?#

IPv4 prefixes#

• 192.0.2.0/24
• 198.51.100.0/24
• 203.0.113.0/24

Soruce: RFC5737

IPv6 prefix#

• 2001:db8::/32
• 3fff::/20

Soruce: RFC3849 RFC9637

Domain names#

• example.com
• example.net
• example.org

Soruce: RFC2606

AS numbers#

• 64496 - 64511 (16Bit)
• 65536 - 65551 (32Bit)

Soruce: RFC5398