Allied Telesis AT-S25 - Manual (Page 246)

Section III: Web Browser Management
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7. To add or remove tagged and untagged ports from the VLAN, modify
the Untagged Ports and Tagged Ports fields next to each module. You
can specify the ports individually (e.g., 2,5,11), as a range (e.g., 5-10),
or both (e.g., 3,7,11-15,17).
Note
Removing an untagged port from the Default_VLAN without
assigning it to another VLAN will leave the port as an untagged
member of no VLAN.
8. Click Apply.
Untagged ports that are added to a VLAN are automatically
removed from their current VLAN assignment. Untagged ports
that are removed from a VLAN are returned to the Default_VLAN.
The modified VLAN is now ready for network operations.