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RFC 2606 / RESERVED DOMAIN NAMES

Choose the suffix that matches the purpose

02

The four reserved TLDs have different intended uses. They are not interchangeable labels for a private network.

DIAGRAMRead RFC section 2 ↗
RFC 2606 · § 2EXAMPLE
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Choose the suffix that matches the purposeFour independent purpose choices: lab.test for DNS testing, api.example for documentation, missing.invalid for an obviously invalid constructed name, app.localhost for traditional loopback use. Each state changes the name, purpose and corresponding semantic object. No DNS query or request is performed.Loopback useSame hostapp.localhostpurpose-specific reservationLoopback returns to this hostLocal does not mean trusted
04
Keep .localhost for loopback use.

app.localhost illustrates a loopback name. Do not point it at another device or assume a loopback service is safe or authorized just because it is local.

Source for this drawing: RFC 2606 · Read RFC section 2 ↗

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RFC 1034 DNS RFC 1918 Private IPv4 addresses

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KEY POINT

.test is recommended for testing current or new DNS-related code; .example is recommended for documentation and examples.

  • .test is recommended for testing current or new DNS-related code; .example is recommended for documentation and examples.
  • .invalid is intended for obviously invalid constructed names. .localhost is reserved for the traditional loopback use; a loopback address returns to the same host.
  • These purposes come from RFC 2606. RFC 6761 later specifies how different software layers should handle the names; reservation does not mean every suffix receives the same answer.
DETAILS

The four scenes are independent choices, not sequential DNS queries. The names are plain text, not active links or requests. Current API and caching-layer distinctions are explained separately in the resolver chapter.

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All chapters in RFC 2606

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For full details, tables, algorithms, and exceptions, read the original RFC.

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