If you have more than one site in a Windows 2000 domain, you need to configure one or more subnets.
To configure a subnet:
1. Use the Active Directory Sites and Services MMC snap-in.
2. Double-click the Sites container to expand it.
3. Double-click the Subnets container.
4. On the Action menu, press New Subnet.
5. Enter the IP Address and Mask.
6. Select the site object for this subnet.
7. Press OK.
See Knowledge Base articles:
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