# Hostname Key: `technology.internet.hostname` Fully qualified domain name or simple hostname. Does not include protocol or path. Allows alphanumeric characters, hyphens, and dots. Subdomains supported. Page: Samples: `example.com`, `www.google.com`, `subdomain.example.org`, `localhost` ## Background ### Origin RFC 952, the DoD Internet Host Table Specification, was published in October 1985 by K. Harrenstien, M. Stahl and E. Feinler of SRI.[[1]](https://www.rfc-editor.org/rfc/rfc952.txt) RFC 952 describes itself as the official specification of the format of the Internet Host Table.[[1]](https://www.rfc-editor.org/rfc/rfc952.txt) Under RFC 952, a name is a text string of up to 24 characters drawn from the letters A to Z, the digits 0 to 9, the minus sign and the period.[[1]](https://www.rfc-editor.org/rfc/rfc952.txt) RFC 952 required the first character of a name to be a letter and the last character not to be a minus sign or a period.[[1]](https://www.rfc-editor.org/rfc/rfc952.txt) RFC 952 makes no distinction between upper and lower case in a name.[[1]](https://www.rfc-editor.org/rfc/rfc952.txt) RFC 1035 says domain name labels must follow the rules for ARPANET host names: start with a letter, end with a letter or digit, and have only letters, digits and hyphens inside.[[2]](https://www.rfc-editor.org/rfc/rfc1035.txt) RFC 1035 limits labels to 63 octets or less and names to 255 octets or less.[[2]](https://www.rfc-editor.org/rfc/rfc1035.txt) RFC 1035 says that two names with the same spelling but different case are to be treated as if identical.[[2]](https://www.rfc-editor.org/rfc/rfc1035.txt) RFC 1123 requires host software to handle host names of up to 63 characters and says it should handle host names of up to 255 characters.[[3]](https://www.rfc-editor.org/rfc/rfc1123.txt) ### Use RFC 2181 states that the DNS itself places only one restriction on labels, which relates to the length of the label and of the full name.[[4]](https://www.rfc-editor.org/rfc/rfc2181.txt) RFC 3696 notes that a trailing period denotes the root, so that a.b.c and a.b.c. are equivalent.[[5]](https://www.rfc-editor.org/rfc/rfc3696.txt) Strip a single trailing dot before comparing or validating hostnames, so that the two spellings of one name land together. (Meridian's view) Lower-case hostnames before joining or deduplicating them, since the same host can arrive as WWW.Example.COM in one file and www.example.com in another. (Meridian's view) RFC 1123 warns that, because a host name segment may begin with a digit, a segment could legally be entirely numeric.[[3]](https://www.rfc-editor.org/rfc/rfc1123.txt) RFC 1123 says host software should check a string syntactically for a dotted-decimal number before looking it up in the DNS.[[3]](https://www.rfc-editor.org/rfc/rfc1123.txt) RFC 3696 says there is an additional rule that essentially requires top-level domain names not to be all-numeric.[[5]](https://www.rfc-editor.org/rfc/rfc3696.txt) RFC 5890 defines the A-label as the ASCII-Compatible Encoding form of an internationalised label.[[6]](https://www.rfc-editor.org/rfc/rfc5890.txt) RFC 5890 says every A-label begins with the prefix xn--.[[6]](https://www.rfc-editor.org/rfc/rfc5890.txt) RFC 6761 reserves the name localhost and names under .localhost as special.[[7]](https://www.rfc-editor.org/rfc/rfc6761.txt) RFC 6761 says users may assume that address queries for localhost names will always resolve to the loopback address.[[7]](https://www.rfc-editor.org/rfc/rfc6761.txt) ### Where you'll meet it The Tranco research ranking publishes a standard list of one million domains.[[8]](https://tranco-list.eu/) Tranco also offers a version of its list with subdomains.[[8]](https://tranco-list.eu/) Tranco says its file uses the same format as the Alexa and Cisco Umbrella lists.[[8]](https://tranco-list.eu/) ## Sources 1. [DOD Internet Host Table Specification](https://www.rfc-editor.org/rfc/rfc952.txt) — standard 2. [Domain Names - Implementation and Specification](https://www.rfc-editor.org/rfc/rfc1035.txt) — standard 3. [Requirements for Internet Hosts -- Application and Support](https://www.rfc-editor.org/rfc/rfc1123.txt) — standard 4. [Clarifications to the DNS Specification](https://www.rfc-editor.org/rfc/rfc2181.txt) — standard 5. [Application Techniques for Checking and Transformation of Names](https://www.rfc-editor.org/rfc/rfc3696.txt) — standard 6. [Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework](https://www.rfc-editor.org/rfc/rfc5890.txt) — standard 7. [Special-Use Domain Names](https://www.rfc-editor.org/rfc/rfc6761.txt) — standard 8. [A research-oriented top sites ranking hardened against manipulation - Tranco](https://tranco-list.eu/) — dataset