Why do DNS changes propagate between 24 & 48 hours?
Each domain has at least two nameservers assigned and these nameservers contain records for the domain. However, if another server anywhere in the world asks the DNS record name servers, it will normally keep a local copy of the DNS record, so the next time you need to know which IP address the domain will resolve to, you will already have the response. This is called caching and ISPs, for example, use it to accelerate network response times.
Once changes are made to the actual DNS records on the two name servers assigned to your domain, those changes are gradually filtered to the other servers in the world so everything is up to date, and this process usually takes between 24 and 48 hours.