Data type Range Default value
REG_SZ Subnet mask (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 DHCP server. If an attempt to locate a DCHP server fails, TCP/IP configures the interface by using an IP address from the Microsoft reserved Class B network (, subnet mask During autoconfiguration, TCP/IP continues its attempts to locate a DHCP server and abandons the autoconfiguration process if one is located.

IPAutoconfigurationMask also appears in the Tcpip\Parameters subkeys. The value of the entry in the Tcpip\Parameters subkey applies, by default, to all interfaces. However, if this entry appears in any Interfaces\<interface-name> subkey, its value overrides the value in the Tcpip\Parameters subkey when configuring that interface.

Note Image Note

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

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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