Hostname Validator
Runs in your browserCheck whether a hostname is valid, label by label, with IDN/Punycode conversion and clear notes on edge cases like underscores and wildcards. Runs locally.
How to read the results
The overall verdict reflects standard hostname rules (RFC 952/1123): each label is 1–63 characters, uses letters, digits and hyphens, and does not start or end with a hyphen; the whole name is at most 253 characters. IDN names are converted to their ASCII (Punycode) form. Notes explain any label that is valid for DNS but not a standard hostname label.
Common issues
- Entering an email address or full URL — enter just the hostname.
- A label longer than 63 characters, or a total length over 253.
- Leading/trailing hyphens in a label.
DNS names vs hostnames
DNS record names (SRV, DKIM, DMARC) legitimately use underscores. This tool distinguishes them from invalid hostname characters.
Validation and IDN conversion run in your browser. Learn about our data sources →
Frequently asked questions
What is the difference between a hostname and a domain?
A domain is a name registered in the DNS hierarchy (e.g.
example.com). A hostname identifies a specific host, often a subdomain (e.g. www.example.com). Every label must follow length and character rules.Are underscores allowed?
Not in standard hostname labels, but they are valid and common in DNS record names like
_dmarc, _domainkey and SRV records. We flag them with an explanatory note rather than a hard error.What is a trailing dot?
A trailing dot (e.g.
example.com.) denotes a fully-qualified name relative to the DNS root. It is valid and simply made explicit.