Note Creating delegated subnetted reverse lookup zones is not a trivial task. It is important to understand how DNS zones work before attempting to create subnetted reverse lookup zones. There are numerous notes throughout this document to which you should pay close attention. It is recommended that you first attempt these procedures in a test environment before deploying them on a live network because of the ease with which mistakes can occur during configuration.

The rapid growth of the Internet community created the need to subnetwork full IP networks into smaller portions. In a subnetted environment, DNS servers can easily delegate authority of forward lookup zones because they are independent of the underlying subnetted infrastructure. However, because of the inverse structure of reverse lookup zones and their strict reliance on the specific subnet structure, delegation of these zones requires special considerations. The Internet Engineering Task Force (IETF) has created RFC 2317, "Classless IN-ADDR.ARPA Delegation," which discusses these considerations.

Delegating subnetted reverse lookup zones complements the ability to delegate forward lookup zones. This flexibility in zone ownership allows you, as the administrator of a parent domain, to delegate control of both a child subdomain and a corresponding subnet of addresses to another administrator. Conversely, as the administrator of a child domain, you now have the control necessary to make changes to both DNS host (A) records or IP address (PTR) records without having to make a request for change through the parent domain.

This article discusses how to configure delegated subnetted reverse lookup zones for a Microsoft Windows DNS server.

Note Simply because your network environment is subnetted does not imply that your DNS server must be configured in the manner described in this article. Creating delegated subnetted reverse lookup zones is an administrative choice only; it is not solely dictated by the underlying subnetted infrastructure.

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A "classful" IP addressing scheme is one that does not break an IP network down into smaller segments. For example, a class C address of 192.168.1.0 with a subnet mask of 255.255.255.0 is a classful IP addressing scheme.

A "classless" IP addressing scheme is one that uses a subnet mask to divide an IP address into smaller segments. For example, a class C address of 192.168.1.0 with a subnet mask of 255.255.255.192 is a classless IP addressing scheme. Along with this network, you would also have the following IP network addresses: 192.168.1.64, 192.168.1.128, and 192.168.1.192.

When subnetting IP networks, additional bits are taken from the host portion of the IP address and given to the network portion. This is defined by adding additional bits to the subnet mask. The value 11111111.11111111.11111111.00000000 shows us a classful subnet mask for a Class C network of 255.255.255.0, while the value 11111111.11111111.11111111.11000000 illustrates the classless subnet mask of 255.255.255.192. Therefore, from the example above, we know that:

If the subnet mask is   The subnet mask bit-count is
   ---------------------   ----------------------------
   255.255.255.128          25
   255.255.255.192          26
   255.255.255.224          27
   255.255.255.240          28
   255.255.255.248          29
   255.255.255.252          30
   255.255.255.254          31

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The syntax

Delegated subnetted reverse lookup zones can be used to transfer administrative control between any parent and child IN-ADDR.ARPA zone in the DNS. Common configurations involve an ISP (Parent) delegating to a Customer Site (Child) or a Corporate Headquarters (Parent) delegating to a Corporate Remote Site (Child). Because the ISP scenario is most typical, it will be used in the following example.

When creating classless reverse lookup zones, you may use notation such as the following: For example: This indicates that the subnetted reverse lookup zone is the 64 subnetwork that is using 26 bits for its subnet mask.

Note If you will be performing any Zone Transfers, between parent and child you need to check the syntax of the files that will be transferred between DNS servers. Not all versions of DNS servers will support the various syntax methods defined in the RFC (the hyphen, the slash, etc.). Microsoft DNS will support any of these methods.

Note Whichever syntax is chosen in the Parent domain MUST be identical to the syntax used in the Child domain.

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The checklist

Filling out the following checklist will make walking through this document easier.

Parent Checklist                         Child Checklist
 ----------------                         ---------------
 <Parent DNS server name>                 <Child DNS server name>
 <Parent DNS server IP>                   <Child DNS server IP>
 <subnet mask>                            <subnet mask>
 <subnet><syntax><subnet mask bit count>  <subnet><syntax><subnet  mask bit 
                                          count>

Here is the example we will use of an ISP who has taken a Class C range and subnetted it into 4 subnets by using the 255.255.255.192 subnet mask. The 4 subnets are 192.168.100.0, 192.168.100.64, 192.168.100.128 and 192.168.100.192. The subnet being delegated to the Customer Site is the second range, that is the 64 network using 65-126 for the host IP addresses.

Parent Checklist       Child Checklist
   ----------------       ---------------
   NS.microsoft.com       NS1.msn.com
   192.168.43.8           192.168.100.126
   255.255.255.192        255.255.255.192
   0-26                   64-26
   64-26
   128-26	
   192-26

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The parent walkthrough for Windows 2000 and Windows Server 2003 environments

Launch the DNS MMC (Microsoft Management Console).Under view, change from standard view to advanced. Highlight Reverse Lookup Zones, right click and select new zone. Select Zone Type of Active Directory Integrated or Standard Primary, click next. Type in either the non-subnetted network ID (for example, 192.168.100) or the reverse lookup zone name (for example, 100.168.192.in-addr.arpa) for the non-subnetted class C address, click next. If you selected standard primary, you can either create a new zone file or if there is an existing zone file, you can place it in the %systemroot%\winnt\system32\dns directory and the server will read it from that directory. Once the primary parent zone is created, right click on the newly created zone, and select new delegation. Add the naming convention you choose as the parent for the delegated child zone, (for example, 64-26). Be sure to communicate that naming convention to the administrator of the child domain. See examples. Add the CNAME (ALIAS) RR (resource records) for the devices within each of the subnets. For example:Note Dynamic updates for subnetted reverse lookups do not work in Windows 2000. The records will need to be added manually. Using "Create Associated PTR record" checkbox will not work for the subnetted reverse lookup zone when "A" (host) record is created through GUI.

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The parent walkthrough for Windows NT 4.0 environments

Note The Microsoft DNS Manager can be used to set up the reverse lookup zone for that name server, as well as the subnetted reverse lookup zone or zones. After the in-addr.arpa zone and the subnetted in-addr.arpa zone(s) are created, the files will need to be manually edited to include the NS, CNAME, and PTR records in each zone file.

Note Several prerequisites are assumed in this example. It is assumed that the Microsoft DNS server has been installed and that the TCP/IP properties (IP Address, Subnet Mask, Default Gateway, and so on) have been configured correctly.

1.	Apply the latest Microsoft Windows NT Service Pack.
2.	Restart your computer when prompted.
3.	Click Start, select Programs, select Administrative Tools, and then click DNS Manager.
4.	On the DNS menu, click New Server, type the IP address or host name of your DNS server, and then click OK.
5.	Create the non-subnetted reverse lookup zone using the following steps: a. Click your DNS server, and then click New Zone on the DNS menu. b. Click the Primary radio button in the Creating New Zone dialog box, and then click Next. c. Type 100.168.192.in-addr.arpa in the Zone Name text box, and then press TAB. d. The Zone File text box should automatically be populated with 100.168.192.in-addr.arpa.dns. e. Click Finish.
6.	When you have finished creating the zones, stop the DNS Server using either of the following methods: • Click Start, point to Settings, click Control Panel, and then double-click the Services icon. Select Microsoft DNS Server in the Service list and click Stop. • Type the following at a command prompt and press Enter: NET STOP DNS Note It is important to stop the DNS service before editing the Zone files or you may lose manually recorded information.
7.	With a text editor, open the non-subnetted reverse lookup zone file that you have just created. We now need to add an NS record that will delegate a subnet to the child DNS server. Add the following to the end of the file: ; Begin Delegation comments ; <subnet><syntax><subnet mask bit count> NS <Child DNS server name> ; End delegation Our example will look like this: ; Begin Delegation sub-zone: 64-26.100.168.192.in-addr.arpa. ; 64-26 NS NS1.msn.com. ; End delegation
8.	It is now necessary to create a CNAME records for each addresses in the delegated subnetted range. Our example looks like this: 65 CNAME 65.64-26.100.168.192.in-addr.arpa. 66 CNAME 66.64-26.100.168.192.in-addr.arpa. 67 CNAME 67.64-26.100.168.192.in-addr.arpa. 68 CNAME 68.64-26.100.168.192.in-addr.arpa. 69 CNAME 69.64-26.100.168.192.in-addr.arpa. ... 126 CNAME 126.64-26.100.168.192.in-addr.arpa. Note The ellipse, "...", indicates the unique IP addresses and hosts between 67 and 126. Ellipses are not valid in the file.
9.	By repeating steps 7 and 8, you may delegate any additional subnetted zones.
10.	After the NS and CNAME records have been entered, save and exit the file.
11.	Start the DNS server using one of the following methods: • Click Start, point to Settings, click Control Panel, and then double-click the Services icon. Select Microsoft DNS Server in the Service list and click Start. • Type the following at a command prompt and press Enter: NET START DNS

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The child walkthrough for Windows 2000 and Windows Server 2003 environments

1.	Launch the DNS MMC (Microsoft Management Console).
2.	Under view, change from standard view to advanced.
3.	Highlight Reverse Lookup Zones, right click and select new zone.
4.	Select Zone Type of Active Directory Integrated or Standard Primary, click next.
5.	Select the option for the "Reverse lookup zone name". Type in the name of the reverse lookup zone, (for example, 64-26.100.168.192.in-addr.arpa) for the subnetted class C address. Be sure to use the naming convention supplied by the administrator of the parent domain, click next.
6.	If you selected standard primary, you can either create a new zone file or if there is an existing zone file, you can place it in the %systemroot%\winnt\system32\dns directory and the server will read it from that directory.
7.	Manually add your PTR (pointer records) as you would any reverse lookup zone. For example: 65 PTR host65.msn.com
8.	You may have to configure the child DNS server(s), which are hosting the delegated zone, to forward to the parent DNS servers. This enables the child DNS servers to resolve records in the zones hosted by the parent DNS servers.

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The child walkthrough for Windows NT 4.0 environments

1.	Apply the latest Microsoft Windows NT Service Pack.
2.	Restart your computer when prompted.
3.	Click Start, select Programs, select Administrative Tools, and then click DNS Manager.
4.	On the DNS menu, click New Server, type the IP address or host name of your DNS server, and then click OK.
5.	Create a subnetted reverse lookup zone using the following steps: a. Click your DNS server, and then click New Zone on the DNS menu. b. Click the Primary radio button in the Creating New Zone dialog box, and then click Next. c. Depending on the syntax chosen at the Parent, select one of pairs listed below. For our example we'll type "64-26.100.168.192.in-addr.arpa" (without the quotation marks) in the Zone Name text box, and then press Tab. Zone Name: 64-26.100.168.192.in-addr.arpa Zone File: 64-26.100.168.192.in-addr.arpa.dns or Zone Name: 64/26.100.168.192.in-addr.arpa Zone File: 64.26.100.168.192.in-addr.arpa.dns or Zone Name: 64.26.100.168.192.in-addr.arpa Zone file: 64.26.100.168.192.in-addr.arpa.dns or Zone Name: 64.100.168.192.in-addr.arpa Zone file: 64.100.168.192.in-addr.arpa.dns or Zone Name: Subnet64.100.168.192.in-addr.arpa Zone file: Subnet64.100.168.192.in-addr.arpa.dns or Note Microsoft DNS Administrator will automatically populate the File Name field when creating zones. If you use the "/" syntax, please be sure to change the filename and replace the "/" character because the underlying file system will not allow a "/" in the file name. Simply substitute the slash character in the filename with another character such as the one suggested in the second example above (64.26.100.168.192.in-addr.arpa.dns). d. The Zone File text box should automatically be populated with 64-26.100.168.192.in-addr.arpa.dns. e. Click Finish. f. Repeat steps a through e, for any additional subnets to being delegated to you.
6.	When you have finished creating the zones, stop the DNS server using either of the following methods: • Click Start, select Settings, click Control Panel, and then double-click the Services icon. Select Microsoft DNS Server in the Service list and click Stop. • Type the following at a command prompt and press Enter: NET STOP DNS Note It is important to stop the DNS service before editing the Zone files or you may lose manually recorded information.
7.	Open the subnetted reverse lookup zone file using a text editor. It is now necessary to create the PTR records for each address in the delegated subnetted range. Add the following to the end of the file: 65 PTR host65.msn.com. 66 PTR host66.msn.com. 67 PTR host67.msn.com. ... 126 PTR host126.msn.com. Note The ellipse, "...", indicates the unique IP addresses and hosts between 67 and 126. Ellipses are not valid in the file.
8.	After the PTR records have been entered, save and exit the file.
9.	Restart the DNS server using one of the following methods: • Click Start, point to Settings, click Control Panel, and then double-click the Services icon. Select Microsoft DNS Server in the Service list and click Start. • Type the following at a command prompt and press Enter: NET START DNS
10.	Hosts on the Internet should now be able to perform a reverse lookup for IP addresses in the delegated reverse lookup zone. One last series of steps is required in order for hosts that use the Customer Site DNS to be able to perform the reverse lookups correctly. It is necessary that a copy of the non-subnetted zone be present on the child domain DNS server. The easiest way to do this is to become a secondary zone to the ISP. Create the secondary zone using the following steps: a. Click your DNS server, and then click New Zone on the DNS menu. b. Click the Secondary radio button in the Creating New Zone dialog box. c. For Zone: enter 100.168.192.in-addr.arpa and for Server: enter the <Parent DNS server IP>. For our example, it is 192.168.43.8. Click Next. d. For Zone Name: enter 100.168.192.in-addr.arpa and for Zone File: enter 100.168.192.in-addr.arpa.dns. Click Next. e. In the IP Masters field, again enter the <Parent DNS server IP>. For our example it is 192.168.43.8. Click Add, Click Next, and then Click Finish.
11.	You may have to configure the child DNS server(s), which are hosting the delegated zone, to forward to the parent DNS servers. This enables the child DNS servers to resolve records in the zones hosted by the parent DNS servers.

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Sample zone files

Parent subnetted reverse lookup zone file

;
;  Database file 100.168.192.in-addr.arpa.dns for 100.168.192.in-addr.arpa zone.
;      Zone version:  4
;

@                       IN  SOA NS.microsoft.com. administrator.microsoft.com. (
                        	4           ; serial number
                        	3600        ; refresh
                        	600         ; retry
                        	86400       ; expire
                        	3600      ) ; minimum TTL

;
;  Zone NS records
;

@                       NS	NS.microsoft.com.

;
;  Zone records
;

;
;  Delegated sub-zone:  64-26.100.168.192.in-addr.arpa.
;
64-26                   NS	NS1.msn.com.
;  End delegation

65                      CNAME	65.64-26.100.168.192.in-addr.arpa.
66                      CNAME	66.64-26.100.168.192.in-addr.arpa.
67                      CNAME	67.64-26.100.168.192.in-addr.arpa.
...
126                    CNAME	67.64-26.100.168.192.in-addr.arpa.

Note The ellipse, "...", indicates the unique IP addresses and hosts between 67 and 126. Ellipses are not valid in the file.

Child subnetted reverse lookup zone file

;
;  Database file 64-26.100.168.192.in-addr.arpa.dns for 64-26.100.168.192.in-addr.arpa zone.
;      Zone version:  1
;

@                       IN  SOA NS1.msn.com. administrator.msn.com. (
                        	1           ; serial number
                        	3600        ; refresh
                        	600         ; retry
                        	86400       ; expire
                        	3600      ) ; minimum TTL

;
;  Zone NS records
;

@                       NS	NS1.msn.com.

;
;  Zone records
;

65                      PTR     host65.msn.com.
66                      PTR     host66.msn.com.
67                      PTR     host67.msn.com.
...
126                    PTR     host126.msn.com.

Note Again, in the above examples, the ellipses indicate the omitted IP addresses between 67 and 126. Ellipses are not valid in the file.

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For additional information about DNS, please see the following white paper available on the Microsoft anonymous ftp server:

File Name: Dnswp.exe
Location: Title: "DNS and Microsoft Windows NT 4.0"

For more information about Classless IN-ADDR.ARPA Delegation, see RFC 2317. For more information about how to obtain this RFC, click the following article number to view the article in the Microsoft Knowledge Base:

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