ICANN, the Internet Assigned Numbers Authority, is the organization charged with overseeing the name and number systems of the Internet. The ICANN authorizes and accredits registrars for maintaining and servicing the registration process of domains names under each of the top-level domains (TLDs). The registry is responsible for maintaining the database of names registered within the TLDs they administer. The registry receives registration information from each domain name registrar authorized to assign names in the corresponding TLD and publishes the information using a special service, the whois protocol.
The top-level domain (TLD) is the last part of an Internet domain name, the letters that follow the final dot of any domain name. For example, in the domain name www.example.com, the top-level domain is .com.The second-level domain (SLD) names are the names directly to the left of the TLDs. For example, in the domain www.example.com, example is the second-level domain and .com is the top-level domain.
According to RFC3696, which defines the format for DNS names and email addresses, the labels (words or strings separated by periods) that make up a domain name must consist of only the ASCII [ASCII] alphabetic and numeric characters, plus the hyphen. No other symbols or punctuation characters are permitted, nor is blank space. If the hyphen is used, it is not permitted to appear at either the beginning or end of a label. There is an additional rule that essentially requires that top-level domain names not be all-numeric.…
A DNS label may be no more than 63 octets long. This is in the form actually stored; if a non-ASCII label is converted to encoded “punycode” form (see Section 5), the length of that form may restrict the number of actual characters (in the original character set) that can be accommodated. A complete, fully-qualified, domain name must not exceed 255 octets.
So, a domain name / label can have from 1 to 63 characters, so the shortest domain name you can have is 1 character and the longest domain name is 63 characters long.
In 1993, ICANN reserved all single-letter and single letter / digit second-level domain names in the Generic Top-Level Domains (gTLD) liks .com, .net, and .org. But there are still a few domains with single letters which had been registered before the restrictions were imposed.
Unfortunately, these domains do not offer email service for the public.
For the Country Code Top Level Domains (ccTLD), the domain registrar for the country decides the minimum length of the domain name.
Many ccTLD supports single letter / digit second-level domain names including:
Again, unfortunately, these domains do not offer email service for the public.
At present, the shortest known domains that provides email service to the public is in.com & rti.in (both, 5 letters & one dot). But, at in.com the username must be at least four letters long, whereas, in rti.in, there is no such restriction. There are others like in.com, but similar restrictions apply. That means:
rti.in provides the World's Smallest Email ID for the public for free (eg: firstname.lastname@example.org or email@example.com etc.).