At one point or two in your life as a webmaster you will need to change IP address for one or all of the following reasons:

1. Changed your webhost.
2. Migrated from shared to dedicated hosting or VPS
3. Changed site IP address to a dedicated one so as to use SSL or for any other reason

DNS changes are instant and some ISP's pick the changes up in minutes. Some others take hours, some take days. It could be frustrating to wait for hours or days for your new DNS changes to take effect and the frustration could get worse if the site is running on both the old server (old IP) and the new server (new IP). That's some bunch of inconsistencies in the offing because an ISP may be toggling requests between both IPs... in other words, you make a change on the site.... you check the site back in 5 minutes and you don't see the change. Stuff like that.

'Nuff background story, now the juice:
On Windows and Linux you can tell browsers the exact IP address you want them to point to when you request a particular site. This can be done by editing the host file.

For example:
I have a site dualcore.com and have recently changed the IP address.

Site: dualcore.com
Old IP: 111.111.111.111
New IP: 222.222.222.222

Once the site has been moved to the new IP I will have to put a notice on the site (on the old IP/location) saying we have moved and then plead with them to wait for propagation OR they can edit their host file to reflect the change at once.

How to edit the host file to reflect the change

For Windows users:
In Windows, the host file can be found at C:\windows\system32\drivers\etc\hosts (if your primary hard drive uses a different drive letter, such as D:, use that drive letter instead). Open the file with Notepad.exe

Go to the bottom of the file, and add the following:


222.222.222.222 dualcore.com
222.222.222.222 www.dualcore.com


Save the file.
Now you can access dualcore.com on the new location.
This has to be done on all computers that will be accessing dualcore.com so its wise to either mail your members with this steps or place it on the homepage of the site on its old location.

Remember to remove these entries from the host file after a few days or a week.

For Linux users:
Search Google, if you were nerdy enough to use Linux then you should be nerdy enough to figure the whole thingy out yourself.
Tip: /etc/hosts

It usually lasts just a couple of minutes when i propagate a domain.

^^ For the first time (immediately after registration before sending any requests to the domain), yes its immediate for everyone.
For subsequent DNS changes on existing domains, it won't be immediate on all ISPs... it won't be immediate on any Nigerian ISP. Etisalat picks up first, MTN picks up last in my experience.