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Configuring Reverse Lookup Zones to Support Classless IP Addresses

Classless address schemes (CIDR; see Chapter 2) are increasingly the rule rather than the exception when IP address ranges are assigned to organizations. Reverse lookups have always presented a special problem with classless addressing because reverse lookup domains in the Internet namespace assume a class-based address scheme with netids having lengths of 8, 16, or 32 bits.

My small organization has been assigned a /27 address space carved out of a Class C address range. Until recently, I could only support reverse lookups for my IP addresses by relying on my ISP to support the reverse lookup zone. This is a problem because most (if not all) ISPs don't allow Windows to make Dynamic DNS updates to zones on their DNS servers.

But, suppose that the ISP is willing to delegate authority to my name servers for reverse lookups on my IP addresses. Is there a way to configure reverse lookup zones based on classless IP addresses? Yes, there is, and it's a pretty straightforward extension of the subdomain concepts just discussed.

Suppose that an organization has been assigned the IP address range, which permits the organization to use IP addresses through with a subnet mask of How does the organization configure the reverse lookup zone on its own DNS servers? Here is the procedure:

  1. Create a reverse lookup zone for the classful IP address range that contains the CIDR address range. In this case, you would create a reverse lookup zone for the netid 192.168.5, using procedures described earlier in the chapter.

  2. Right-click the class-based reverse lookup zone in the DNS Management console and choose New Domain from the context menu.

  3. In the New Domain dialog box, specify the classless address range using the notation <subnet>/<network mask length> wherein

    • <subnet> is the starting IP address in the address range, and

    • <network mask length> is the number of bits in the netid portion of the subnet mask.

    For example, you would enter 64/26 to indicate a subnet starting on IP address with a 26-bit subnet mask. (The first three octets of the address range are established by the reverse lookup zone itself.)

  4. Click OK.

Figure 3.48 shows the DNS Management Console after the subdomain has been established.

Figure 3.48 Reverse lookup has been configured for the address range 192.168.5/26.


The procedure for automatically creating PTR RRs while creating Host Address RRs does not work when the IP address falls in a classless range defined in this way. You will need to create the PTR RRs manually.

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