Registered domain names can consist only of ASCII letters, numbers and the hyphen (-) - the same character set as all other hostnames. The full stop (dot, .) is used to separate DNS labels.

Since the presently permit character range does not allow for the use of other characters commonly found in non-English languages, and does not allow multi-byte characters necessary for most Asian languages, the Internationalized domain name (IDN) system has been developed and is now in testing stage for a group of top-level domains established for this purpose.

The underscore character is frequently used to ensure that a domain name is not recognized as a hostname, as with the use of SRV DNS server records, for example, although some older systems such as NetBIOS did allow it. To avoid confusion and for other reasons, domain names with underscores in them are sometimes used where hostnames are required.

Domain names are often referred to simply as domains and domain name registrants are frequently referred to as domain owners, although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use.