EDNS Client Subnet

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EDNS Client Subnet or ECS is a DNS extension that allows a recursive DNS resolver to specify the network subnet for the host on which behalf it is making a DNS query. This is generally intended to help speed up the delivery of data from content delivery networks, by allowing better use of DNS-based load balancing to select a service address serving the content expected to be hosted at that domain name, when the client computer is in a different network location from the recursive resolver.[1] [2] The EDNS client subnet mechanism is specified in RFC 7871.

As of 2019, DNS resolution service providers supporting EDNS Client Subnet include Google Public DNS, OpenDNS, and Quad9.[3]


  1. ^ "How it works". A Faster Internet. Retrieved 2018-03-27.
  2. ^ "EDNS Client Subnet (ECS) Guidelines | Public DNS | Google Developers". Google Developers. Retrieved 2018-04-02.
  3. ^ "Which CDNs support edns-client-subnet?". Retrieved 2018-04-02.