EDNS Client Subnet
An EDNS0 option that lets a recursive resolver pass a truncated client IP prefix to authoritative servers so CDNs can return geographically appropriate answers.
EDNS Client Subnet (ECS) is the option that lets a recursive resolver forward the client's network prefix (typically /24 for IPv4, /56 for IPv6) to upstream authoritative servers. CDNs and GeoDNS providers use it to pick a nearby edge instead of optimising for the resolver's own location, which would send a Comcast user behind an 8.8.8.8 lookup to whichever PoP is close to Google. ECS trades a small amount of client-network privacy for much better CDN geo-routing. Some privacy resolvers (Cloudflare 1.1.1.1) deliberately strip ECS; others (Google 8.8.8.8) forward it.