Data type


Default value


Subnet mask, in dotted decimal format


Stores the subnet mask assigned to the client by the IP autoconfiguration feature.

Autoconfiguration allows TCP/IP to configure an interface even when it cannot locate a Dynamic Host Configuration Protocol (DHCP) server. If TCP/IP cannot locate DCHP server, it configures the interface by using IP addresses from the Microsoft reserved Class B network (, subnet mask During autoconfiguration, TCP/IP continues trying to locate a DHCP server, and it abandons autoconfiguration if it finds one.

The IPAutoconfigurationMask entry can also appear in the interface-name subkeys. By default, the entry in the Parameters subkey applies to all interfaces. However, if this entry appears in an interface-name subkey, that entry takes precedence over the entry in the Parameters subkey when configuring that interface.

Note Image Note

This entry is used only when IP autoconfiguration is enabled, that is, when the value of the IPAutoconfigurationEnabled entry is 1.

Windows 2000 does not add this entry to the registry. You can add it by editing the registry or by using a program that edits the registry.

Caution Image Caution

Do not change the value of this entry. If you do, the IP address assigned to the interface might not be reliable.

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