Limits
| Type | Length | Characters |
|---|---|---|
| email addresses | 64 characters before the @, 255 characters after that, maximum total length of 256 ref | All ASCII characters, with backslash quoting for special characters |
| subdomains | 63 octets per subdomain, but the total domain name has a length limit of 253 ref | Letters, digits, and hyphens are allowed. Case-insensitive. Cannot start with a hyphen. Some systems allow underscores _, but most don't. ref |
Implementation-defined limits
- The length of a URL: can be as low as 2,083 characters in Internet Explorer, but Firefox displays URLs up to 65,536 characters.
- HTTP Headers: Apache sets a limit of 8k and Nginx limits to 8k as well.
- Emails: are limited to 20-25MB.