Subdomain tracking issues
Subdomains under the same registrable domain (blog.example.com, shop.example.com) typically share a first-party cookie set on the parent domain, so a user stays continuous. Problems arise when the cookie domain is scoped too narrowly, when subdomains use separate properties, or when one subdomain appears as a referrer to another. This page distinguishes subdomain handling from true cross-domain tracking.
What this means
GA4 and most first-party tags set the analytics cookie on the highest registrable domain, so all subdomains of example.com share one identifier automatically — no cross-domain linker is needed for same-registrable-domain subdomains.
The failure mode is when a tag scopes the cookie to a single subdomain, or when each subdomain is wired to a different property, so the same person is counted twice.
Subdomain vs cross-domain
True cross-domain tracking (with the `_gl` linker) is only required when the destination is a different registrable domain. For subdomains of one domain, the right fix is usually cookie domain configuration and referral exclusion, not the cross-domain linker.
- Same registrable domain: shared parent-scoped cookie
- Different registrable domain: needs cross-domain linker
- Add subdomains to referral exclusions to avoid self-referrals
How it appears in analytics and logs
If one subdomain shows another as a referrer, the shared cookie is not being read across them — the cookie domain scope or property setup is wrong.
Diagnostic use case
Decide whether a multi-subdomain site needs cross-domain configuration or simply a correctly scoped cookie, and spot subdomain self-referrals.
What WebmasterID can help detect
WebmasterID can reveal a subdomain appearing as a referrer in its own reports, the clearest sign that subdomain cookie scope is fragmented.
Common mistakes
- Scoping the analytics cookie to a single subdomain.
- Treating subdomains as needing the cross-domain linker.
- Running separate properties per subdomain without intent.
Privacy and accuracy notes
Subdomain continuity relies on a first-party cookie scoped to the parent domain, not personal identifiers. WebmasterID treats subdomains of one property as one site by default.
Related pages
- Cross-domain tracking issues
A single user crossing from one domain to another (site to a separate checkout or booking host) should stay one user and one session. Without cross-domain tracking, the second domain starts a fresh session and often a self-referral, double-counting users and breaking attribution. This page explains how the GA4 linker passes the client ID and the common reasons it does not arrive.
- Self-referrals and lost attribution
A self-referral is when your own site shows up as a referring source in your reports. It usually means a session was broken and a new one started attributed to your domain, often when a visitor crosses subdomains or returns from a payment provider. Self-referrals fragment sessions and steal credit from the real source. This page explains the causes and the fix.
- Hostname leakage across properties
Your measurement ID is visible in page source, so anyone can paste it on another site and have that traffic report into your property. Staging copies, scraped clones, and proxies do this too. The leaked hits inflate and pollute your data with another domain's traffic. This page explains hostname/property leakage and the valid-hostname filtering that contains it.
- Multi-Site Analytics
Keep subdomains and sites correctly separated.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.