Changing WAN providers

My problem is this. We currently use BellSouth with an ip range 66.x.x.x  and we are switching to a new wan provider with a range of 64.x.x.x . I have a pix firewall with a website and email sever on the inside. Obviously the current DNS at BellSouth is set to point to the current ip's for my website and mail server. When we switch the website and mail server will be at new ip addresses.

I have heard it can take up to 72 hours for the new DNS to propagate throughout the net. So does this mean my website and mail will fail when I reprogram my firewall to the new ip range, if so how do I avoid having them go down during that 72 hour period.
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nprignanoConnect With a Mentor Technical ArchitectCommented:
If your new ISP is a major telco provider using backbone-connected DNS servers, your DNS replication will NOT take more than 4-6 hours (worst case 12, but that's absolute worst case).  This said, arrange for this switchover to occur at night and you will avoid any disruptions in service where service is needed.

The only way to prevent this is to have both IP addresses routed to the same server, but this is unneccesary if your website is not a on-demand application.

the best thing you can do is estimate when the change will happen.  I doubt it will take the full 72hours more like 2 or 3 hours depending on how long the name is cached in the other systems that are out of your controll.

I have been using and it changes amazingly fast within a couple hours so maybe something for you to think about.

captonyAuthor Commented:
I was hoping you guys would have a sneaky way to avoid the out outage, but in the end I bit the bullet and made the change late at night. It took about 6 hours with the Website and mail was a little bit longer. Thanks for you efforts.
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