Static IP configuration UI error

[jamiebegin]

When setting up a gateway with a static IP address, the netmask textbox is prefilled with "255.255.255.0." Only it's not actually filled with a value, but rather an input hint. This caused me to configure the device without a netmask, which took some time to troubleshoot.

Correct fix: Add input validation on the form.

Easy fix: Remove the input hint so it's obvious that the value is null.