The Internet uses a special domain to support gateway location and Internet address to host mapping. Other classes may employ a similar strategy in other domains. The intent of this domain is to provide a guaranteed method to perform host address to host name mapping, and to facilitate queries to locate all gateways on a particular network in the Internet.
Note that both of these services are similar to functions that could be performed by inverse queries; the difference is that this part of the domain name space is structured according to address, and hence can guarantee that the appropriate data can be located without an exhaustive search of the domain space.
The domain begins at IN-ADDR.ARPA and has a substructure which follows the Internet addressing structure.
Domain names in the IN-ADDR.ARPA domain are defined to have up to four labels in addition to the IN-ADDR.ARPA suffix. Each label represents one octet of an Internet address, and is expressed as a character string for a decimal value in the range 0-255 (with leading zeros omitted except in the case of a zero octet which is represented by a single zero).
Host addresses are represented by domain names that have all four labels specified. Thus data for Internet address 10.2.0.52 is located at domain name 126.96.36.199.IN-ADDR.ARPA. The reversal, though awkward to read, allows zones to be delegated which are exactly one network of address space. For example, 10.IN-ADDR.ARPA can be a zone containing data for the ARPANET, while 26.IN-ADDR.ARPA can be a separate zone for MILNET. Address nodes are used to hold pointers to primary host names in the normal domain space.
Network numbers correspond to some non-terminal nodes at various depths in the IN-ADDR.ARPA domain, since Internet network numbers are either 1, 2, or 3 octets. Network nodes are used to hold pointers to the primary host names of gateways attached to that network. Since a gateway is, by definition, on more than one network, it will typically have two or more network nodes which point at it. Gateways will also have host level pointers at their fully qualified addresses.
Both the gateway pointers at network nodes and the normal host pointers at full address nodes use the PTR RR to point back to the primary domain names of the corresponding hosts.
For example, the IN-ADDR.ARPA domain will contain information about the ISI gateway between net 10 and 26, an MIT gateway from net 10 to MIT's net 18, and hosts A.ISI.EDU and MULTICS.MIT.EDU. Assuming that ISI gateway has addresses 10.2.0.22 and 188.8.131.52, and a name MILNET- GW.ISI.EDU, and the MIT gateway has addresses 10.0.0.77 and 184.108.40.206 and a name GW.LCS.MIT.EDU, the domain database would contain:
10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 18.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 26.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 220.127.116.11.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 18.104.22.168.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 22.214.171.124.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 126.96.36.199.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU. 188.8.131.52.IN-ADDR.ARPA. PTR A.ISI.EDU. 184.108.40.206.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU.
Thus a program which wanted to locate gateways on net 10 would originate a query of the form QTYPE=PTR, QCLASS=IN, QNAME=10.IN-ADDR.ARPA. It would receive two RRs in response:
10.IN-ADDR.ARPA. PTR MILNET-GW.ISI.EDU. 10.IN-ADDR.ARPA. PTR GW.LCS.MIT.EDU.
The program could then originate QTYPE=A, QCLASS=IN queries for MILNET- GW.ISI.EDU. and GW.LCS.MIT.EDU. to discover the Internet addresses of these gateways.
A resolver which wanted to find the host name corresponding to Internet host address 10.0.0.6 would pursue a query of the form QTYPE=PTR, QCLASS=IN, QNAME=220.127.116.11.IN-ADDR.ARPA, and would receive:
18.104.22.168.IN-ADDR.ARPA. PTR MULTICS.MIT.EDU.
Several cautions apply to the use of these services: