Why does updating a domain's nameservers take so long? [closed]

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When I change my domain's (a dot com) nameservers it can take up to 2 days.
I know that that's usual, but WHY is that?

Just curious because my webhosting went down and I have to wait now because I changed nameservers, why?

EDIT:
Did some little research and my webhosting says it can MAXIMUM take up to 24 hours.
Ok, I knew it was around 24 - 48 hours, but it is apperently only 24 hours.
Anyways, I found this article: http://www.tech-faq.com/how-to-flush-dns.html
And it says that you can flush DNS with ipconfig blabla, also already knew that.
But what is remarkable is that this is standing at the second paragraph:

The MaxCacheTtl represents the maximum time that a DNS lookup%u2019s results will be cached for. The default value is 86,400 seconds. If this value is set to 1, DNS entries will only be cached for a single second.

Now take your calculator and type in: 86,400 / 60 / 60 = TADAH!! 24 (hours)
I haven't tried it yet because the registery key wasn't present in my registery :S

Answer 1

When you change your nameservers you have to ensure that the old nameservers also contain the new information, otherwise some resolvers won't notice that the delegation in the .com zone has changed.

For more information see https://serverfault.com/questions/322490/what-is-the-correct-behaviour-when-nameservers-change/322524#322524

Answer 2

First of all, I think the 2 days figure is outdated. I've seen it take UP TO 48 hours, but standard now is more like 4-8 hours. Also, because of the nature of DNS, your computer may see the dns resolve hours before (or after) your phone does.

Anyways, the answer to your question boils down to the concept of propagation. The DNS server you update has to tell all the other DNS servers out there about the change. The other servers only request an update once the TTL has passed, and those numbers can all add up when you're talking about all the DNS servers around the world.