Prestel
In 1983, the Prestel messaging service known as “Prestel Mailbox” was launched, initially hosted on the computer known as "Enterprise", and later available from all IRC computers by means of a centralised messaging computer known as "Pandora". This facility extended the original day one concept of “Response Frames” whereby an end user could send a message back to the IP who owned the page via special pages, for example to order goods or services. The user’s name, address, telephone number, and date could be added automatically to the message when the IP set up the response frame by means of codes which triggered extraction of key data from the users account held on the IRC computer. Initially response frames were ingathered by an IP from each IRC individually, but later the facility to collect messages from all IRCs at the UDC from where they could be ingathered centrally was implemented, and with the introduction of Mailbox, they could be retrieved from any IRC.
IPv6
In the Domain Name System (DNS), hostnames are mapped to IPv6 addresses by AAAA resource records, so-called "quad-A" records. For reverse resolution, the IETF reserved the domain ip6.arpa, where the name space is hierarchically divided by the 1-digit hexadecimal representation of nibble units (4 bits) of the IPv6 address. This scheme is defined in .
At the design stage of the IPv6 DNS architecture, the AAAA scheme faced a rival proposal. This alternate approach, designed to facilitate network renumbering, uses "A6" records for the forward lookup and a number of other innovations such as "bit-string labels" and "DNAME" records. It is defined in and its references (with further discussion of the pros and cons of both schemes in ), but has been deprecated to experimental status ().