{
  "guide": "track-subdomains-ga4",
  "checked": "2026-11-11",
  "rule": "same property, same web data stream, same Measurement ID on every subdomain",
  "cross_domain_needed": false,
  "cross_domain_exception": "only when different cookie domains cause self-referrals",
  "cookie_domain": "default \u2014 written at the highest shared (root) domain",
  "worked_example": {
    "subdomains": [
      "www",
      "blog",
      "shop"
    ],
    "subdomain_count": 3,
    "measurement_ids": 1,
    "recomputed": "3 subdomains : 1 Measurement ID"
  },
  "self_referral_sign": "the site's own domain appears as a referrer",
  "split_in_reports": "Hostname dimension",
  "do_not": [
    "create a stream per subdomain",
    "add subdomains to cross-domain measurement by habit",
    "scope the cookie to one subdomain"
  ]
}