To help speed up DNS Resolution lots of ISPs implement DNS Caching.
This means that when you visit a website like Hostiname.us your browser will ask your ISPs DNS servers for the IP Address which hosts Hostiname.us.
If someone else has already asked the ISP for the IP address for the same domain recently, they may have a cache of the result and will simply return that instead of passing the request on to the DNS servers responsible for your domain to save time.
Most ISPs respect the TTL on your DNS which is usually between a few hours and 24hrs, some ISPs however don't respect this and may cache the DNS result for up to 48hrs.
We usually find that DNS caching issues resolve themselves within a few hours, and almost always within 12hrs, this does mean that when you make DNS changes it can take a little while for the change to fully take effect.
If you're keen to get going, you can bypass this wait by updating your hosts file - check out the related articles for a step by step guide on how to do this.
