- Published on
Domain Name Length Limits
- Authors
- Name
- hwahyeon
According to the DNS standard, domain names have strict length limits:
| Part | Limit | Notes |
|---|---|---|
| Full domain name | Up to 253 octets in ordinary text form | The DNS wire format allows up to 255 octets. A trailing root dot may make the displayed form 254. |
| Each label | Up to 63 octets | A label is each part separated by dots, such as www, example, and com. |
Why 253 and not 255?
RFC 1035 limits the DNS wire-format representation of a full domain name to 255 octets.
In the wire format:
- Each label is preceded by a one-octet length field.
- The name ends with a zero-length label representing the DNS root.
- The dots used in textual notation are not stored as literal dots in the wire format; they correspond to the boundaries between labels.
As a result, the maximum length of the usual textual form without a trailing root dot is 253 octets. If the final root dot is explicitly included, the textual form can be 254 octets long.
Implication
- DNS does not define a separate fixed limit on the number of labels.
- However, the total name-length limit restricts how many labels a domain name can contain.
- No individual label may exceed 63 octets.
- For example,
a.b.c.d.e.f.example.comis valid as long as every label and the full domain name remain within their respective limits.