.dd part of the Federal Republic of Germany, which had already been assigned the ccTLD .de. The ISO 3166-1 code "DD" was withdrawn in 1990. .dd .dd was the assigned country code top-level domain (ccTLD) for the German Democratic Republic (East Germany). It was chosen based on the ISO 3166-1 alpha-2 code for the German Democratic Republic, the letters coming from the German name of the country: '. In accordance with IANA policy, .dd was therefore available to be assigned as the country code top-level domain (ccTLD) for East Germany. However, this was never done, and so .dd was never added
Top-level domain Dynamic Delegation Discovery System, and e164.arpa for telephone number mapping based on NAPTR DNS records. For historical reasons, arpa is sometimes considered to be a generic top-level domain. RFC 6761 reserves the following four top-level domain names to avoid confusion and conflict. Any such reserved usage of those TLDs should not occur in production networks that utilize the global domain name system: RFC 6762 reserves the use of .local for link-local host names that can be resolved via the Multicast DNS name resolution protocol. RFC 7686 reserves the use of .onion for the self-authenticating names of Tor hidden services. These