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When you scrub a dataset, replace sensitive values with reserved or clearly fictional ones that keep the shape your tests need. Don’t null them out. Use the documentation IP blocks, the reserved example domains, and the published example phone range. Use fictional street addresses. None of these is guaranteed to pass every application’s validator, so check the behavior your test depends on.
Reserved values at a glance
| Field | Safe example or range | Intended use and caveat |
|---|---|---|
| IPv4 | 192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24 |
Documentation blocks (TEST-NET-1, -2, -3) from RFC 5737. They should not appear on the public Internet and are not for local use. |
| IPv6 | 2001:db8::/32 |
The RFC 3849 documentation range, as given in Google’s developer documentation style guide. |
| DNS test name | .test |
Recommended for testing DNS-related code (RFC 2606). |
| Documentation name | .example; example.com, example.net, example.org |
Reserved for documentation and examples (RFC 2606). |
| Visibly invalid name | .invalid |
For domain names intended to be obviously invalid (RFC 2606). |
| Local host | .localhost |
Reserved for localhost use (RFC 2606). |
e.g. [email protected] |
Google’s guide recommends reserved example domains with generic example addresses. | |
| US phone | 800-555-0100 to 800-555-0199 |
Google’s US example range. It is not an international range and does not make every 555 number safe. |
| Street address | Fictional addresses | Google’s guide supplies samples such as 1800 Amphibious Blvd. (Mountain View), Avenida da Pastelaria, 1903 (Lisbon) and 8 Rue du Nom Fictif (Paris). |
Why reserved values beat nulls and made-up values
A null or empty string breaks tests that parse, format, join on or validate the field. A random realistic value risks pointing at a real host, person or place. Reserved values avoid both problems. RFC 5737 states that “Three IPv4 unicast address blocks are reserved for use in examples in specifications and other documents.” Google’s style guide is blunt about phones: “Never use a real phone number in examples.”
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Choosing per field
IP addresses
Seed from the three RFC 5737 blocks for IPv4 and 2001:db8::/32 for IPv6. Two mistakes to avoid:
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- Don’t treat RFC 1918 private ranges (such as
10.0.0.0/8) as documentation ranges. They are a different thing. RFC 5737 says its blocks are not for local use, so they suit fixtures and examples, not a LAN you actually configure. - Don’t invent public-looking addresses because they “probably aren’t used”. The documentation blocks exist so examples don’t collide with addresses assigned for other purposes.
Domains and email
The RFC 2606 labels do different jobs, so pick by what the fixture must do:
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- Must look like an ordinary example:
example.com,.netor.org, or the.exampleTLD. - Exercising DNS-related code:
.test. - Must be visibly bad:
.invalid, for negative-path tests. - Must refer to the local machine:
.localhost.
For emails, a generic local part on a reserved domain ([email protected]) keeps the format valid while keeping scrubbed mail from reaching a real inbox. Give each record a distinct local part if uniqueness constraints apply.
Phone numbers
Use 800-555-0100 through 800-555-0199 for US-format data. Its scope is US only. Another source, RFC 3966, shows a different example, 555-0100 through 555-0149 in the 212 area code. The mismatch shows why you should copy a specific documented range rather than assume all 555 numbers are fake. For other countries, no universal range was established in these sources, so look for that country’s regulator-published example numbers.
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Street addresses
Avoid real addresses. Google’s fictional samples above cover US, Portuguese and French formats, which is handy for testing address layout and character handling.
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What these ranges do not cover
- Validators: The guidance is written for examples. A library may reject
.testaddresses,555numbers or documentation IPs. Run the values through your actual validation and integration path. - Names, government IDs, payment data: The sources establish no universal reserved range. Don’t invent one. For payment integrations, use your provider’s current official test fixtures.
- Connected environments: A local unit test, a connected integration test and a documentation snippet have different needs. Reserved names behave differently when something actually resolves them.
A quick decision checklist
- State what the test checks: format, uniqueness, resolution, rejection, or display.
- Pick the reserved value matching that purpose from the table.
- Keep the format (length, separators, character set) the code expects.
- Run the value through the real validator and confirm it behaves as intended.
- Confirm nothing in the test can resolve, dial, mail or ship to the value.
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