Why Do DNS Changes Take 72 Hours?

I mentioned yesterday that once you change DNS settings it can take up to 72 hours for the website to become available. But why?

Well, it's not just DNS settings that can have this affect. For example, if I move a website from Windowos to Linux, or Windows without ASP to Windows with ASP, then the same delay can occur.

It's not always 72 hours. I have seen it take place as quick as an hour, and over the last couple of years at most 24 hours. But what's happening?

Basically, the DNS settings are passed to the domain controller (the company with overall controll of the high level, e.g. .com, .co.uk etc). This tells them where to look for your website. The actuall address of the machine is also passed to them.

Now every time you type in a URL your ISP has to look up from this list where to go for that website. If every time a URL was entered they had to go to the master list, there would be a lot of unnecessary traffic to and fro quering this list. So instead each ISP takes a copy of this list at regular intervals. This might be once a day or whatever, but no less than once every 72 hours.

In my experience, most ISPs do collect it every day, so 24 hours is the worst you can expect to wait. I've noticed that .co.uk names do seem to become available quicker than .com, so maybe my ISP collects the .co.uk list even more often than that.

But that's why you have to wait 72 hours when you change your DNS records...