01
Choose a mode for the actual field
Domain requires two labels. Hostname accepts a local label such as printer. FQDN here checks an explicit final root dot and also accepts com.; it does not assume that a DNS search suffix will be added.
DNS & Domains
Domain name validator
Check bare names, one per line. Choose the rule set before interpreting the result.
Domain name validator
Names, one per line
Validation report
Names are checked in this browser; no DNS lookup or visit to the entered domain is made.
Validation options
At least two labels; a final dot is optional. No TLD list lookup.
UTS #46 nontransitional · STD3 · NFC · Bidi and joiner checks.
Up to 100 nonempty lines; 65,536 UTF-16 units total and 4,096 per line. JSON exports include invalid entries.
Open Punycode / IDN converterInput or mode changed. Validate again before exporting.
A Domain name validator helps when a configuration rejects a name that looks ordinary. A dot, a hidden joiner or the encoded length of an accented label can be the cause. Compare the original spelling with the normalized ASCII name, then follow the reported line and label back to the input.
Why use this Domain name validator
Measure the encoded name
The Domain name validator measures each label after IDN conversion. A short Unicode label can become a longer xn-- label; the report shows that expansion alongside the total DNS wire length. The optional root dot stays visible without consuming the 253-byte body allowance.
Keep the failed rows in view
Batch checks preserve input order, duplicate entries and physical line numbers. One bad row does not hide the good ones. The JSON report includes both outcomes and stable diagnostic codes, so a review can retain the exact evidence instead of silently dropping rejected names.
Choose the intended naming rules
02
Read normalized spelling and byte counts
Open the label details to compare the encoded parts. Fullwidth dots and case can be normalized, while ß is preserved by the nontransitional profile. Validation never replaces the text you pasted.
03
Freeze a report before exporting
Turn automatic validation off to use Validate or Ctrl/Cmd + Enter. Editing immediately disables export until the new check completes. Examples and Clear preview the replacement count; Undo restores the previous input and report together.
What the report measures
One profile for Unicode and ACE input
IDN validation uses the same strict UTS #46 profile for Unicode names and existing xn-- names. It checks STD3 characters, hyphens, Bidi and joiner context. A malformed ACE label cannot pass just because its characters happen to be ASCII.
Lengths that match the encoded form
Each encoded label must contain 1–63 bytes. The dotted ASCII body is limited to 253 bytes and its uncompressed DNS wire representation to 255. These are different counts: the wire representation includes length octets and the terminating root octet.
A hostname validator with batch diagnostics
Empty lines are ignored, but their line numbers remain accounted for. Up to 100 names share the selected mode. The domain syntax checker reports invalid names independently; the first 50 rows appear initially and Show more reveals the rest. Export always includes every row.
Scope of a syntax result
- Syntax does not establish existence
- There is no DNS, WHOIS, RDAP, HTTP or certificate request. No TLD allowlist is consulted, so an unfamiliar suffix can pass syntax. A valid result does not establish registration, availability, reachability, ownership or trust.
- A strict host profile, not all DNS names
- Underscore service labels, wildcards and escaped DNS owner names are rejected. The fixed IDN profile also rejects third-and-fourth-position hyphens in non-ACE labels. This is stricter than a bare ASCII LDH test and does not describe every legal DNS record owner.
- Bare names only
- Outer whitespace is trimmed per line. URLs, ports, paths, email addresses, percent escapes and internal whitespace are rejected instead of extracted or split. Multilabel numeric suffixes are rejected; a single numeric local label is accepted only in hostname mode. A bare 123 is accepted as a local hostname; 123. is rejected because it explicitly names a numeric label directly under the root.
- Explicit FQDN validation
- This mode requires a final dot as an explicit root marker. A name without that dot can still be written as an FQDN in other contexts; it simply does not meet this mode’s contract. A lone root dot and repeated final dots are rejected.
- Bounded input, complete reports
- The input cap is 65,536 UTF-16 units, with 4,096 per line and 100 nonempty lines. Exceeding a batch cap blocks export rather than truncating data. There is no file import, sorting, deduplication or valid-names-only export. JSON retains invalid rows and diagnostic codes.
Domain name validator questions
What is the maximum length of a domain name?
For this host/IDN profile, each encoded label is at most 63 ASCII bytes. The body with dots between labels can use 253 bytes, or 254 characters when the final root dot is included. The DNS wire form uses at most 255 bytes. Unicode character counts alone do not decide these limits.
Can a hostname start with a number?
Yes. A name such as 3com.example passes. Digits can also occupy an intermediate label. A multilabel name ending in an all-numeric label fails this profile; an IPv4 literal therefore does not pass as a hostname. Hostname mode permits a single local numeric label.
What is a trailing dot?
The final dot marks the DNS root and makes the name explicitly absolute. Domain and hostname modes permit it; the explicit FQDN mode requires it. Only one final dot is accepted. Dots between empty labels, as in a..example, remain errors.
Can a hostname be a single label like localhost?
Yes, in hostname mode. Domain mode deliberately requires at least two labels. Choosing hostname does not prove that localhost or printer resolves on your machine; local resolver and search-domain behavior are outside this report.
Why can a name load in my browser yet fail a hostname check?
Browsers parse URLs and may apply different acceptance rules. This tool only checks a bare name using its published strict IDN profile. It does not remove a port or path, permit underscores, or test a connection. A browser loading something is evidence of a different operation, not a reason to hide a syntax failure.
Have another domain case to check?
Review your names before configuring them
Use the Domain name validator to inspect the actual encoded labels and keep a report of the names your configuration accepts or rejects.