Domain Name Rules & Restrictions: What You Can Register (2025)

Quick Answer

Table of Contents

Universal Character Rules

The Domain Name System (DNS) enforces strict length limits defined in RFC 1035 and clarified in subsequent RFCs. These limits apply to every domain name on the internet, regardless of TLD.

Label Length: 1-63 Characters

A "label" is each section of a domain name separated by dots. In www.example.com:

• www is a label (3 characters)
• example is a label (7 characters)
• com is a label (3 characters)

Technical specification:

Most TLDs require a minimum of 3 characters for second-level domains, though some allow shorter:

• .de (Germany): Allows 1-character domains (e.g., x.de)
• .io: Allows 2-character domains
• .com: Requires minimum 3 characters
• .uk: Allows 2-character domains at third level only

Example of maximum length:

this-is-a-very-long-domain-name-that-reaches-the-sixty-three.com

This domain has exactly 63 characters in the second-level label (before .com).

Total Domain Name: 253 Characters Maximum

The complete domain name (all labels plus dots) cannot exceed 253 characters in its human-readable form.

Why 253 and not 255?

The DNS protocol allows 255 octets total, but:

• Each label requires a length prefix byte
• The domain ends with a null byte for the root
• Formula: 255 = (1+63)+(1+63)+(1+63)+(1+63)+1
• This equals four 63-character labels plus three dots = 253 printable characters

Practical example:

subdomain.subdomain2.subdomain3.example.com

You could theoretically chain labels up to the 253-character limit, though this is rare in practice.

Character Case: Case Insensitive

DNS treats uppercase and lowercase letters identically:

• Example.com = EXAMPLE.COM = example.com
• All resolve to the same destination
• Registrars typically store everything in lowercase
• Display case may vary but has no technical effect

Allowed Characters in Domain Names

Standard domain names use a limited character set often called "LDH" (Letters, Digits, Hyphens).

Letters (a-z)

• All 26 letters of the English alphabet
• No accented characters in standard domains
• International characters require IDN/Punycode (see below)

Numbers (0-9)

• All digits allowed anywhere in the domain
• Numeric-only domains permitted: 123.com
• Mixed alphanumeric common: web2.com, 24seven.com
• Number positioning unrestricted (unlike hyphens)

Hyphens (-)

Hyphens have specific placement rules:

Why the 3-4 position restriction?

The pattern xn-- at the start of a label indicates Punycode encoding for internationalized domain names. Registries block double-hyphens in positions 3-4 to prevent confusion with IDN encoding.

Characters NOT Allowed

The following cannot appear in standard domain names:

Prohibited Patterns

Beyond individual character rules, certain patterns are universally prohibited.

Starting or Ending with Hyphen

Invalid examples:

• -example.com (starts with hyphen)
• example-.com (ends with hyphen)
• -example-.com (both)

Why prohibited: The DNS protocol uses hyphens as word separators, not as terminators. Edge hyphens could create parsing ambiguity.

Double Hyphens in Positions 3-4

Invalid examples:

• ab--cd.com
• xy--domain.com
• 12--34.com

Why prohibited: The xn-- prefix is reserved for Punycode/IDN encoding. If you could register ab--cd.com, it might conflict with internationalized domains or confuse DNS resolvers.

Valid double hyphens (other positions):

• a--bcd.com (positions 2-3)
• abc--d.com (positions 4-5)
• my--domain.com (positions 3-4 but with "my" prefix making it positions 4-5)

All-Numeric TLD Restriction

While domains can be all-numeric, TLDs cannot consist only of numbers. This is an ICANN policy for new gTLDs to avoid confusion with IP addresses.

Internationalized Domain Names (IDN)

Internationalized Domain Names allow non-ASCII characters---like Chinese, Arabic, Cyrillic, or accented Latin characters---through a system called Punycode.

How IDN Works

IDN converts international characters to ASCII-compatible encoding (ACE):

The xn-- prefix signals Punycode encoding to DNS systems.

IDN Length Calculation

Important: The 63-character limit applies to the Punycode form, not the display form.

This means:

• A short display domain could exceed limits after conversion
• Each non-ASCII character may convert to multiple ASCII characters
• Always verify final Punycode length before registering

IDN Character Restrictions

Not all characters work in all TLDs:

1. Script restrictions: Many TLDs limit which scripts can be used
2. Mixed script blocking: Combining Cyrillic and Latin often blocked
3. Confusable characters: Characters resembling Latin letters may be restricted
4. Language tables: Each TLD publishes allowed character sets

Browser IDN Display

Modern browsers protect against homograph attacks (look-alike domains):

Example homograph attack:

• Real: apple.com
• Fake: аpple.com (Cyrillic "а")
• Browsers show: xn--pple-43d.com

Reserved and Restricted Names

Even if a domain meets all technical requirements, registries and ICANN reserve certain names from registration.

IANA Reserved Domains

The Internet Assigned Numbers Authority (IANA) maintains permanently reserved domains per RFC 2606 and RFC 6761:

These can never be registered by anyone.

Registry Infrastructure Names

Most registries reserve labels used for their own services:

• nic.tld - Registry information center
• whois.tld - WHOIS service
• www.tld - Sometimes reserved
• mail.tld - Mail services
• dns1-4.tld - DNS infrastructure
• registry.tld - Registry operations

ICANN-Required Reservations

All new gTLDs must reserve:

1. ICANN names: icann, iana, internic
2. Two-letter codes: Country codes blocked at second level
3. Special-use labels: Per IETF registry
4. Name collision names: Terms causing technical conflicts

Premium Strings

Registries designate certain domains as "premium" with higher registration prices:

Common premium categories:

Premium pricing varies wildly by registry. Check actual pricing before assuming availability means affordability.

Blocked Terms

Registries may block terms for various reasons:

1. Offensive language: Profanity, slurs, hate speech
2. Regulated industries: Medical, legal, financial terms (some TLDs)
3. Trademark protection: Famous brands (varies by TLD)
4. Government terms: Official titles, agency names
5. Misleading terms: False claims about affiliation

TLD-Specific Registration Requirements

Beyond universal rules, individual TLDs impose unique requirements. Here are the most common restricted TLDs.

.US - United States Nexus Requirement

The .us TLD requires demonstrable connection to the United States.

Nexus categories (must select one):

Enforcement:

• Registry conducts weekly random audits (approximately 0.5% of new registrations)
• Registrants have 30 days to respond to verification requests
• Non-response results in domain deletion without refund

Additional .us restrictions:

• No WHOIS privacy allowed (legal requirement)
• Registrant must maintain nexus throughout ownership
• Business presence must be legitimate and ongoing

.EU - European Union Presence

The .eu TLD requires EU establishment or citizenship.

Eligible registrants:

Post-Brexit note: UK residents lost .eu eligibility after Brexit. UK-held .eu domains were suspended unless registrants demonstrated EU presence.

Trademark exception: Non-EU trademark holders can register via licensing arrangements with EU-based licensees.

.DE - Germany Administrative Contact

Germany's .de domain has unique contact requirements.

Key requirements:

For non-German registrants:

• Must provide German administrative contact
• Can use registrar's trustee service
• German agent listed as admin-c
• Registrant remains legal owner

Technical enforcement:

• DENIC (German registry) verifies nameserver activity
• Domains without active nameservers deleted after 30 days
• No refund for technical non-compliance deletions

.AU - Australian Business Requirements

Australia restricts .au domains to verified Australian entities.

Domain-specific requirements:

ABN/ACN explained:

• ABN: Australian Business Number (11 digits)
• ACN: Australian Company Number (8 digits)
• Required for commercial .au domains

Ongoing requirements:

• ABN/ACN must remain active for renewal
• Domain name must match or relate to business name
• auDA (registry) enforces eligibility throughout domain life

.CA - Canada CIRA Requirements

Canadian .ca domains require Canadian presence.

Eligible registrants:

• Canadian citizens (anywhere in world)
• Permanent residents
• Canadian corporations
• Canadian trademark holders
• Associations registered in Canada

Other Restricted TLDs

Why Domains Show "Unavailable" When Not Registered

You search for a domain, WHOIS shows no owner, yet it's marked "unavailable." Here's why this happens.

1. Registry Reserved Names

The registry has reserved the name but not registered it:

• Infrastructure names (nic, whois, dns)
• Premium names held for auction
• Geographic or trademarked terms
• Names on collision lists

How to check: Look up registry policies for your target TLD.

2. Pending Delete Status

The domain is in the deletion lifecycle:

Domains in pending delete show no WHOIS owner but cannot be registered until the cycle completes.

3. Sunrise/Launch Period

New TLDs go through launch phases:

1. Sunrise: Trademark holders only
2. Landrush: Early access (premium pricing)
3. General Availability: Open registration

During sunrise, domains may show unavailable to general registrants.

4. Name Collision Blocks

ICANN blocks certain names that could conflict with internal network usage:

• Common internal hostnames
• Corporate intranet names
• Terms causing DNS resolution issues

These names may never become available.

5. Premium Pricing Not Displayed

Some registrars show "unavailable" for premium domains rather than displaying high prices. Try:

• Different registrar
• Direct registry lookup
• Premium domain marketplaces

6. Technical Registration Status

Use DomainDetails.com to check detailed domain status via RDAP.

Best Practices for Domain Registration

Before Registration

1. Check all character rules - Verify your domain contains only allowed characters
2. Measure label lengths - Ensure each label is under 63 characters
3. Verify TLD requirements - Research geographic or entity restrictions
4. Test IDN encoding - For international characters, check Punycode length
5. Research premium status - Confirm pricing before getting attached to a name

During Registration

1. Use accurate WHOIS data - False information can result in domain loss
2. Select correct nexus codes - For restricted TLDs like .us
3. Provide required documentation - ABN, trademark numbers, etc.
4. Configure nameservers promptly - Some TLDs require quick activation

After Registration

1. Enable WHOIS privacy - Where available and permitted
2. Set up auto-renewal - Prevent accidental expiration
3. Monitor domain status - Use DomainDetails monitoring for changes
4. Keep contact info current - Required for many TLDs

Frequently Asked Questions

Can I register a 1-character domain name?

It depends on the TLD. Most major TLDs require minimum 2-3 characters. However:

• .de (Germany): Allows single characters
• .ws: Has allowed some single characters
• Most gTLDs: Minimum 3 characters
• .com/.net/.org: Minimum 3 characters

Single-character domains that exist are extremely valuable (often worth millions) and were grandfathered from early registrations.

Why can't I use underscores in domain names?

The underscore character (_) is not part of the DNS hostname specification (RFC 952). While underscores appear in some DNS record types (like SRV records with _tcp), they're invalid in domain names themselves. This is a fundamental DNS protocol restriction, not a registrar policy.

Can domain names be all numbers?

Yes, domain names can consist entirely of numbers, like 123.com or 888888.com. However, TLDs themselves cannot be all-numeric per ICANN policy for new gTLDs. This prevents confusion with IP addresses.

What happens if I register a domain with wrong WHOIS info?

Providing false WHOIS information violates ICANN policy and most registrar terms of service. Consequences can include:

• Domain suspension or deletion
• Loss of dispute protection (you can't prove ownership)
• Transfer to legitimate trademark holder
• Account termination by registrar

How do I find out why a domain is blocked?

1. Check RDAP/WHOIS for status codes
2. Look up registry-specific reserved name lists
3. Search for trademark conflicts
4. Check ICANN name collision lists
5. Contact the registry directly for explanations
6. Use DomainDetails.com for comprehensive status information

Can I register a domain with spaces?

No. Spaces are not allowed in domain names under any circumstances. Use hyphens or concatenation instead:

• Instead of "my domain" use my-domain.com or mydomain.com
• There is no technical workaround for spaces

Are there restrictions on how many domains I can register?

No universal limit exists. However:

• Some TLDs limit registrations per entity
• Registrars may flag unusual bulk activity
• Premium domains may have purchase limits
• Certain restricted TLDs limit per-organization registrations

What's the difference between blocked, reserved, and premium domains?

Key Takeaways

• Label limits are strict: 63 characters maximum per label, 253 total for the complete domain name
• LDH rule: Only letters (a-z), digits (0-9), and hyphens (middle positions only) allowed
• No edge hyphens: Domains cannot start or end with hyphens, and double-hyphens in positions 3-4 are blocked for IDN compatibility
• TLD requirements vary wildly: .us requires U.S. presence, .eu requires EU presence, .au requires Australian business documentation
• "Unavailable" has many causes: Reserved names, pending deletion, premium pricing, or technical holds can all block registration
• IDN uses Punycode: International characters work but convert to ASCII, and length limits apply to the encoded form
• Premium domains exist: Short, dictionary, and desirable terms often cost 10-1000x standard registration prices

Next Steps

Immediate Actions

1. Check your target domain: Use DomainDetails.com to see complete registration status
2. Review TLD requirements: Verify you meet eligibility for restricted TLDs
3. Calculate Punycode length: For international domains, verify encoded length

Related Reading

• How to Choose a Domain Name - Practical selection criteria
• Understanding Domain Extensions - TLD types and purposes
• Country Code TLDs Guide - Geographic domain options
• Premium Domains Explained - Understanding premium pricing

Research Sources

• RFC 1035 - Domain Names Implementation and Specification - Original DNS specification with 63-octet label limit
• RFC 1123 - Internet Host Requirements - Domain name character requirements
• RFC 5891 - IDNA Protocol - Internationalized domain name specification
• IANA Reserved Domains - Official list of reserved example domains
• ICANN Reserved Names Policy - New gTLD reservation requirements
• Namecheap .US Domain Requirements - US nexus requirements
• EURid .EU Rules - European domain requirements
• auDA .AU Domain Rules - Australian domain eligibility
• DENIC .DE Requirements - German domain admin contact rules
• Dynadot Domain Name Rules - Character and format requirements

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