AT-AR220E Broadband Router Installation Guide
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Subnet Mask: Specifies the local subnet mask of the LAN interface in the
router. The default is 255.255.255.0.
Clicking the Apply button saves your changes in the router.
Note
Configuration changes are automatically saved into the router’s
flash memory within 30 seconds after you click the Apply button,
but are not activated until you reboot the device.