The Public Suffix List (PSL) is a community‑maintained catalogue of domain suffixes that identifies which parts of a domain are registrable and which are controlled by registries. It’s a foundational resource for browsers, security tools, and anything that needs to correctly understand domain boundaries.

✨ Features
📚 Daily‑updated list — synced automatically from GitHub

🌍 Defines public suffixes — identifies which domain segments are registry‑controlled

🧩 Essential for cookie & domain logic — prevents apps from treating registry domains as user‑controlled

🔧 UTF‑8 encoded — consistent and predictable formatting

🔄 Atom feed available — stay informed about list changes

📝 Clear specification — formatting rules documented on the PSL Wiki

🆓 Open source — maintained by the community and Mozilla Foundation

Why it matters
Without the Public Suffix List, software has no reliable way to know where the registry‑controlled part of a domain ends and the user‑controlled part begins. Registries all follow different rules, and there is no algorithmic method to determine the boundary. That creates serious security risks:

🍪 Cookie leakage — without the PSL, a site could set a cookie on .co.uk, exposing it to every domain under co.uk

🛑 Cross‑domain privilege escalation — browsers wouldn’t know which domains should be isolated from each other

🧭 Incorrect domain grouping — history, permissions, and storage could be grouped under the wrong “site”

⚠️ Invalid domain assumptions — static TLD lists quickly become outdated, causing software to misclassify real or retired TLDs

The PSL provides the only authoritative, machine‑readable boundary for determining what counts as a registrable domain. That makes it foundational for browsers, security tools, firewalls, certificate rules, and any system that needs to safely understand domain hierarchy.