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I understand that this is the way a PTR record would be made on a Class A subnet (10.100.250.100 255.0.0.0)

dnscmd /RecordAdd 10.in-addr.arpa. 100.250.100 PTR host.domain.tld

To clarify the syntax, this is what it should be for a Class C subnet (192.168.1.100 255.255.255.0)

dnscmd /RecordAdd 1.168.192.in-addr.arpa. 100 PTR host.domain.tld

Is that right?

Now how do I do this for a host with the IP address 172.31.111.210 on a 172.31.111.192/26 network? I'm not sure how to do this with a classless subnet mask.

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You have to ask the person who transferred control of that block to you. There's more than one way to do it. See RFC 2317 for the basic method. They use a CNAME to a name inside a zone they delegate to you. –  David Schwartz Apr 3 '12 at 10:57
    
@DavidSchwartz: But there are already PTR records for other hosts in the subnet, and I cannot see any CNAME records in the forward lookup zone. –  paradroid Apr 3 '12 at 11:02
    
You would look in the reverse zone for CNAME records. If there are none, ask the person who manages that block to put them in for you. –  David Schwartz Apr 3 '12 at 11:46
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1 Answer

up vote 0 down vote accepted

Given the heirarchal nature of PTR records, a "zonefile" can only contain a C, B, or A block of PTR records. However, RFC 2317 offers a solution to this in short by having the owner of the larger block delegate (by use of ~254 CNAME records) the PTR records in question. Of course, this will require your upstream provider to implement this correctly.

Many upstream providers simply will accept requests for updates to your PTR records. In the case of public address space, I would suggest contacting them to see what their preferred method is.

In either case, within your Windows DNS server, you will manage a single zonefile that contains a series of PTR records, which can be appended to by (e.g.):

dnscmd /RecordAdd 111.31.172.in-addr.arpa. 210 PTR host.domain.tld
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I've updated my original answer to incorporate RFC 2317. Thanks for the information @DavidSchwartz! –  Kyle Smith Apr 3 '12 at 22:38
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