What is cross-domain tracking?
Cross-domain tracking is what keeps a visitor counted as one person with one source when they move between two separate domains you own, say from your marketing site to a booking or checkout domain. Without it, the second domain sees a brand-new stranger.
The problem it solves
Analytics cookies are tied to one domain. So when a visitor crosses from yoursite.com to book.differentdomain.com, the second site cannot read the first's cookie. The journey splits in two, and the original source is lost, which also shows up as a self-referral or a payment-gateway referral.
How it works
Cross-domain tracking passes a small identifier (the linker parameter) in the URL as the visitor crosses, so the second domain adopts the same session and source instead of starting fresh. In GA4 you configure it in the data stream's tagging settings by listing your domains.
Cross-domain vs subdomains
- Subdomains (blog.yoursite.com) are handled by GA4 almost automatically, see subdomain tracking.
- Separate root domains need explicit cross-domain setup, this guide's subject.
If your funnel spans two real domains, this is not optional, it is the difference between attributing sales correctly and losing them to a referral. The hands-on setup is in cross-domain tracking in GA4.
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