New Website: Domain Name Propagation

So you got a new website or you moved your existing website, but your not seeing the new site when you updated your domain name and your being told it can take up to 72 hours to propagate? What is propagation and why does it take up to 72 hours?. Let’s imagine you live in Colorado Springs, CO and you just have changed the nameservers for your domain that is hosted in Salt Lake City, UT. When you open your website in a web browser, your request is not going to the hosting server directly, it has to pass through your ISP and several other ISP nodes first. So your computer starts by checking local DNS cache on your actual device/computer, then the request is sent to your local ISP (say Comcast). From there, the request goes to the upstream hub in Denver, then connects to their backbone. After that, the request is sent to