Amazon CloudFront automatically routes requests for your content to the nearest edge location.
Behind the scenes, it uses the IP address of the DNS resolver that is making the DNS query for the content. This model worked well back in the days when a particular resolver had a single, fixed geographic location. Today, many popular DNS resolvers are geographically dispersed (Google DNS and OpenDNS are two good examples) and the location of a particular resolver is no longer an accurate predictor of the location of the client. This can lead to sub-optimal performance if a request for content is routed to an edge location that is farther away than necessary.
The EDNS-Client-Subnet extension to the DNS protocol solves this problem by returning additional information in response to a DNS query (read about how it works). This information allows the content delivery network to make a better decision. This extension was developed as part of the Faster Internet project.
Today, we are adding EDNS-Client-Subnet support to CloudFront. You don't have to make any changes to your code or to your CloudFront distributions to take advantage of this new feature. Your users will see an immediate benefit if they are using a geographically dispersed DNS resolver that supports the EDNS-Client-Subnet protocol.