A DMZ (demilitarized zone) is a segmented part of a network that is used to host all publicly accessible websites and services. The intention is to protect the internal network from external threats. It is an effective strategy to minimize public exposure of your critical assets as well as limit the damage caused when an intruder is able to penetrate your network. A great definition of a DMZ can be found here.
Having an offline local network with no Internet access can be useful for a number of reasons. A few that come to mind are home lab networks, non-cloud IoT device networks, and “closed circuit” IP security camera network. For my network, I set up a separate offline IP security camera network that I am using as a baby monitor system (the quality is so much better and it is more secure than traditional baby monitors because I can lock down access from outside users).
After I set up VLANs on my home network, I quickly noticed that I could not access the web administration page on my TP-Link T2600G-28TS L2 managed switch (affiliate link) from a different VLAN even though I had allowed access for my PC via a firewall rule. I could access the web admin page of the switch from the same VLAN (the default VLAN1) so I knew the switch was functioning properly.
When I initially created VLANs on my UniFi wireless access points, I was still new to VLANs in general, and I was not quite sure how to configure my network switch. I was wanting to join my wired and wireless devices together on several different VLANs for various purposes (IoT network, guest network, security camera network, etc.). Setting up VLANs on a TP-Link switch is not too difficult once you understand how VLANs work.
A few months ago, I switched to using my own cable modem instead of using the ISP provided modem/router, and I wanted to access the web interface for the cable modem status. Another reason was to change the default password if it had one since default passwords are not secure. The default web address for cable modems is typically http://192.168.100.1. Perhaps that is a common default IP address to avoid potential IP address conflicts with standard consumer grade routers which often default to 192.