Visual RFC.
ALL RFCs / RFC 2606
Read the RFC ↗

RFC 2606 / START HERE

Reserved domain names, before the details

DNS means Domain Name System; TLD means top-level domain. RFC 2606 reserves domain names for examples and testing.

A domain name is a path through the DNS name tree. This Best Current Practice sets aside four top-level names and documents three second-level example domains, so tests and documentation need not borrow someone else’s current or future name.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Name in an example

  2. Reserved domain

  3. Purpose and limits

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

A made-up name that seems unused can later collide with real DNS data. RFC 2606 reduces that naming risk. RFC 6761 later updates it with special-use handling for different software layers; it does not replace the publication.

A familiar example

Use api.example.org in a fictional API manual, lab.test for a DNS experiment, missing.invalid for an invalid-name case and app.localhost for loopback use. These choices do not provide a firewall, prove a service trustworthy or guarantee that every example lookup fails.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

DNSRFC 1034 · beginner overview ↗Private IPv4 addressesRFC 1918 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Avoid collisions between test names and real namesThe Domain Name System (DNS) is a tree of names. A top-level domain, or TLD, is directly below the root. Inventing an unreserved TLD for a lab can collide with global use.
  2. Choose the suffix that matches the purposeThe four reserved TLDs have different intended uses. They are not interchangeable labels for a private network.
  3. Locate the three reserved example domainsexample.com, example.net and example.org are reserved example names at the second level, below their respective TLDs.
  4. Separate API handling from caching-server handlingRFC 6761 updates the reservation with layer-specific handling. A name-resolution API is the application-facing lookup interface; a caching server is a separate DNS role.
  5. Do not turn a naming reservation into a security boundaryChoosing a reserved example name avoids one kind of confusion. It does not make copied code, services or traffic harmless.
Start with chapter one →

This introduction explains RFC 2606 (1999). Read the original RFC ↗