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form fields need a name="..." attribute, e.g. name="firstname", ... if you are just starting out, forget about using the overly complicated and inconsistent mysqli extension. instead, use the much simpler and more modern PDO extension, and use prepared queries. you should also put the form and form processing code on the same page, as this results in the simplest code. the code for any page should be laid out in this general order - 1) initialization, 2) post method form processing, 3) get method business logic - get/produce data needed to display the page, 4) html document.

Most of the time a responsive design moves horizontal elements gradually vertical. Right now you have Device Info and Network Settings in a left column and the settings themselves in a right column. The first responsive step towards a narrower design is to use one column and make the labels on the left be headings - which they might already be. Device Info ======================================= ... Network Settings ======================================= Ethernet Settings --------------------------------------- DHCP [ ] IP Address [__________] Subnet Mask [__________] Default Gateway [__________] DNS [__________] Side note #1: you really ought to support multiple DNS servers. You could keep the navigation anchors, which I assume exists, by moving the left column into a slide-out menu. To go narrower, you take the two columns of setting name on the left and setting value on the right and merge them into a single column of setting name then setting value. Device Info =================== ... Network Settings =================== Ethernet Settings ------------------- DHCP [ ] Enabled IP Address [_________________] Subnet Mask [_________________] Default Gateway [_________________] DNS [_________________] Side note #2: many interfaces for DHCP present it as a DHCP vs. manual/static option, not just a mere checkbox.