Visual Sentinel is a website monitoring platform that helps users track uptime, performance, and changes in real time, while free URL monitoring tools provide basic checking of whether a website is online or offline without cost. In the comparison of UptimeRobot vs Visual Sentinel, UptimeRobot is ideal for simple uptime alerts, whereas Visual Sentinel offers more advanced monitoring like detailed diagnostics and visual change tracking. DNS monitoring ensures that domain records are correctly resolving to servers, and a DNS server not responding error usually means the request cannot be completed due to network or configuration issues. Tools to check site status and web status are essential for quickly identifying downtime, ensuring websites remain accessible and reliable for users.