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.arpa: the backbone of the internet

As the first TLD ever created, .arpa has been fundamental to the development of the modern internet. Learn about this infrastructure domain to deepen your understanding of the DNS ecosystem.


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Although fully integrated and active in the DNS, no one can actually register a domain name under the top-level domain .arpa! This extension is almost exclusively used to manage the internet's technical infrastructure and is part of the operationally critical infrastructure of the DNS. Still today, the stability, integrity and efficiency of the operation of this TLD are a matter of great importance for the whole internet community! Keep reading to learn about the history of .arpa and its function in the modern internet infrastructure.

From ARPANET to .arpa and the creation of the DNS

The internet is one of the most successful and disruptive innovations on the list that can be ascribed to the Defense Advanced Research Projects Agency (DARPA). The United States Department of Defense created the agency to develop and research cutting-edge projects. The Economist recently referred to it as the agency "that shaped the modern world". And this for good reason.

In 1966, DARPA initiated the project ARPANET. It was the first packet-switched network with distributed control and one of the first networks based on the TCP/IP protocol suite, the two technical protocols at the foundation of today's internet. ARPANET is the de-facto precursor of the modern internet, the technology that has changed every aspect of our life over the last twenty years.

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In the early days, the mapping between numerical addresses and the corresponding name in ARPANET was very cumbersome and time-consuming. The search was performed through lookup tables distributed as computer files between network administrators. The .arpa extension was the very first attempt at defining a hierarchical naming system. It represents the initial deployment of the Domain Name System (DNS), which automates the lookup function efficiently through the use of domain names. 
For the first time, the .arpa naming system defined the roles and standards of this naming function and aimed to contain all existent ARPANET hosts at the time before migrating to the future top-level domains. In fact, it should have been phased out slowly and replaced by a new set of domain names for specific types of network members (.com, .org, .edu, .gov, .net and .mil) as stated in RFC 920:

"After a short period of initial experimentation, all current ARPA-internet hosts will select some domain other than ARPA for their future use. The use of ARPA as a top-level domain will eventually cease."

In the end, removing .arpa from the DNS became almost impossible. The extension was fully integrated for the reverse DNS lookup of IP addresses. Furthermore, the email addresses associated with the Network Information Center (NIC), which administers the naming system, were also based on this TLD.

What is .arpa today?

The TLD .arpa works very differently from other generic TLDs. It is not used for naming hosts. Today, IANA administers the domain in cooperation with the internet community at ICANN and under the guidance of the Internet Architecture Board (IAB). In 2000, its name was redefined as a backronym of Address and Routing Parameter Area to reduce confusion about its past role within the ARPANET project and highlight its actual use in the modern internet as an infrastructure domain.

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Today's role of .arpa in the network infrastructure

Instead of being phased out, .arpa was reduced to a few critical functions to support the naming system. These functions depend on various subdomains with a specific purpose. These are some of the most relevant functions of .arpa that domain experts should know about.

IP address mapping

The subdomains in-addr.arpa for IPv4 and ip6.arpa for IPv6 are responsible for reverse DNS lookups of IP addresses described in the dedicated section 3.5 of RFC 1035. Their goal is to provide a guaranteed method to perform host address to hostname mapping and to facilitate queries to locate all gateways on a particular network on the internet. Thanks to the mapping function, you get redirected to the corresponding DNS record if you enter an IP address. This is used for troubleshooting, anti-spam measures and system monitoring. In all these cases, it is helpful to know the domain name. Furthermore, RFC 5855 specifies a naming scheme to delegate the two subdomains to two different sets of nameservers: in-addr-servers.arpa and ip6-servers.arpa.

Telephone number mapping

RFC 2916 defines the use of the DNS for E.164 numbers, which can be used on the internet using the reserved second-level domain e164.arpa. The number is mapped to a domain name composed of the inverse of its digits: for example, the number +34-06-1234567891 becomes 1.9.8.7.6.5.4.3.2.1.6.0.4.3.e164.arpa. The domain e164.arpa is active for the ccTLDs of Austria, Czech Republic, Finland, Germany, Ireland, Lithuania, Poland, Romania, the Netherlands and the United Kingdom.

Residential networking

As stated in RFC 8375, the Internet Engineering Task Force (IETF) reserved the domain home.arpa in 2018 for naming within residential homenets. Domain names ending with home.arpa reference a zone, served locally. Their contents are available only to a particular homenet and a name ending in '.home.arpa.' is not globally unique.

Sinking DNS traffic

In 2015, IAB approved the delegation of AS112 in the .arpa zone. AS112 provides a mechanism for reverse lookups on IP addresses that are not unique. This way, DNS zone administrators can sink traffic relating to parts of the global DNS namespace under their control to the AS112 infrastructure without requiring coordination with the AS112 operators.

.arpa helps the internet in many ways

In its task as an infrastructure domain for the internet, .arpa has many other special-use subdomains and purposes that we have briefly listed here:

  • 6tisch.arpa: to enable IPv6 over the time-slotted channel hopping (TSCH) mode of IEEE 802.15.4.
  • e164.arpa: to map E.164 numbers to internet URIs
  • eap-noob.arpa: to enable the nimble out-of-band authentication method of the Extensible Authentication Protocol framework
  • iris.arpa: to locate internet registry information services
  • uri.arpa and urm.arpa: for the resolution of URIs and URNs.

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