A reverse lookup performs the opposite. It queries DNS for a host name based on an IP address. For example, some e-mail servers perform a reverse DNS lookup on a sending mail server to determine that the host name offered by the remote mail server actually matches the IP address of the SMTP connection attempting to deliver the message.

Windows 2000's DNS service supports both forward and reverse lookup zones. If you want your DNS server to support reverse lookups, you need to create a reverse lookup zone for each subnet you want to support.

To create a reverse lookup zone, follow these steps:

1. Open the DNS console, and expand the server where you want to create the zone.

2. Right-click Reverse Lookup Zones, and choose New Zone to launch the New Zone Wizard.

3. Click Next, choose Standard Primary, and click Next.

4. In the Network ID field, enter the first three octets of the zone's IP address, and click Next.

5. When the wizard offers a name for the DNS zone file, click Next to accept the default name, and click Finish.

After creating the zone, you need to add pointer resource (PTR) records to the zone. These records associate an IP address with a host name. You can create the PTR records explicitly in the reverse zone, or the DNS console can create the PTR records automatically when you create records in the forward lookup zone.

Just because you host your own DNS services doesn't mean that reverse DNS makes it to your DNS servers. Your Internet service provider (ISP) might handle reverse lookup for your subnet. If you're not sure, check with your ISP.