Recently, I saw I introduction on CDN at
http://www.aflexi.net/technology/how-aflexi-cdn-works
I was wondering, why user's web browser will know it need to contact Aflexi DNS Server in Los Angeles, but not DNS Server provided by ISP (Like AOL)?
Thanks!
Your Internet Service Provider will likely push the DNS server information through the DHCP protocol (of course, you can override this information locally on your machine). This is how your machine will get configured with DNS servers to launch queries against.
The way a CDN works is as follows: companies relying on CDN based delivery of their contents will manage their domains through a CDN provider. When a request comes for say domain D, a machine will contact its configured DNS server and will be directed to the "authoritative entity" for the domain D in question. From this point, the CDN DNS server can reply with an answer that provides a binding to an IP address "closest" to where the request originated.
The property "closest" is determined, amongst other things, based on the requesting machine's IP address. It is nonetheless non-trivial to assign a "metric" based on this information: there is no direct correlation between "IP address" and "physical location", vital information for effecting as best as possible contents to the requesting machines.
This usually works by assigning the same IP address to a number of authoritative DNS servers for the domain around the world, and playing some tricks with routing in the internet default-free zone such that every request goes to the closest server (measured in network hops, not miles). This is called 'IP Anycast'. The different servers can then deliver whatever answers the provider wants them to. It's a good solution because network topology matters much more to performance than physical distance.