Question : How do web hosting providers keep public ip addresses live when one ISP fails

I am just curious about how data centers keep hosts available to Internet users, if the ISP goes down that the public ip address belongs to.

So for example if Internet DNS records have www.foo.com pointing to 71.71.71.71, how does the data center keep that ip address live when the ISP providing it goes down?  I know they use multiple ISPs for redundancy, but how do they share the public ip address between ISPs?  Or are they doing something else, like some sort of rollover DNS?

Godaddy for example guarantees 99.99% up time.  So how do they "roll over" the traffic to www.foo.com, to a different ISP for inbound requests?  I imagine there must be some sort of static entries somewhere that show a route to their public ip addresses, on multiple ISPs?

Answer : How do web hosting providers keep public ip addresses live when one ISP fails

... but I have to try :)

In a very basic sense an IP block lives behind an Autonomous System (AS) Number, and you can have mutliple routes to that AS. When a block of IP addresses is allocated to an ISP an AS number is created to allow this to be routed across the Internet.

Border Gateway Protocol (BGP) exists to help figure out routes between different autonomous systems.

Therefore you can have a collection of links for any given AS, and therefore for any given IP address range.

Chris