16.5. Network Names and NumbersThe original DNS specifications didn't provide the ability to look up network names based on a network number -- a feature that was provided by the original HOSTS.TXT file. Since then, RFC 1101 has defined a system for storing network names; this system also works for subnets and subnet masks, so it goes significantly beyond HOSTS.TXT. Moreover, it doesn't require any modification to the name server software at all; it's based entirely on the clever use of PTR and A records.
Remember that to map an IP address to a name in DNS, you reverse the IP address, append in-addr.arpa, and look up PTR records. This same technique is used to map a network number to a network name, for example, to map network 15/8 to "HP Internet." To look up the network number, include the network bits and pad them with trailing zeros to make four bytes, and look up PTR data just as you did with a host's IP address. For example, to find the network name for the old ARPAnet, network 10/8, look up PTR data for 0.0.0.10.in-addr.arpa. You get back an answer like ARPAnet.ARPA.
If the ARPAnet were subnetted, you'd also find an address record at 0.0.0.10.in-addr.arpa. The address would be the subnet mask, 255.255.0.0, for instance. If you were interested in the subnet name instead of the network name, you'd apply the mask to the IP address and look up the subnet number.
This technique allows you to map the network number to a name. To provide a complete solution, there must be a way to map a network name to its network number. This, again, is accomplished with PTR records. The network name has PTR data that points to the network number (reversed with in-addr.arpa appended).
Let's see what the data might look like in HP's zone data files (the HP Internet has network number 15/8) and step through mapping a network number to a network name.
Partial contents of the file db.hp.com:
Partial contents of the file db.corp.hp.com:; ; Map HP's network name to 188.8.131.52. ; hp-net.hp.com. IN PTR 0.0.0.15.in-addr.arpa.
Partial contents of the file db.15:; ; Map corp's subnet name to 184.108.40.206. ; corp-subnet.corp.hp.com. IN PTR 0.0.1.15.in-addr.arpa.
Partial contents of the file db.15.1:; ; Map 220.127.116.11 to hp-net.hp.com. ; HP's subnet mask is 255.255.248.0. ; 0.0.0.15.in-addr.arpa. IN PTR hp-net.hp.com. IN A 255.255.248.0
Here's the procedure to look up the subnet name for the IP address 18.104.22.168:; ; Map the 22.214.171.124 back to its subnet name. ; 0.0.1.15.in-addr.arpa. IN PTR corp-subnet.corp.hp.com.
Now for the bad news: despite the fact that RFC 1101 contains everything you need to know to set this up, there's very little software we know of that actually uses this type of network name encoding, and very few administrators go to the trouble of adding this information. Until software actually makes use of DNS-encoded network names, about the only reason for setting this up is to show off. But that's a good enough reason for many of us.movie.edu. IN PTR 0.249.249.192.in-addr.arpa. IN PTR 0.253.253.192.in-addr.arpa.
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