Public Suffix List (PSL) - Quick Reference

Key Rule: PSL entries CANNOT set cookies

example.com in PSL:

• Who can set cookies: Only subdomains (a.example.com, b.example.com)
• Cookie sharing: None (each subdomain isolated)
• Think: "Each apartment rents independently, no shared lobby"

*.example.com in PSL:

• Who can set cookies:
  • ✅ example.com (shares to ALL descendants)
  • ❌ a.example.com, b.example.com (they're public suffixes)
  • ✅ child.a.example.com, child.b.example.com (but only for themselves)
• Cookie sharing: Everyone reads example.com's cookies, but children can't share with each other
• Think: "Hotel owner controls lobby, guests can't set room rules, but guests' visitors can"

Memory trick:

• No * = Subdomains are independent owners
• With * = Parent owns everything, subdomains are just public spaces (but their children can own again)

Bottom line: PSL creates a "cookie boundary" - determines who gets to host vs who just receives.