URL normalization (also called URL canonicalization) is the process of unifying multiple URL representations that point to the same web page into a single standard form. For example, "http://Example.COM/page/", "https://example.com/page", and "https://example.com/page/index.html" may all point to the same page, but search engines treat them as separate URLs.
The main elements to unify during URL normalization include: protocol (http to https), hostname case (Example.COM to example.com), trailing slash (consistent presence or absence), default port removal (:443 or :80), percent-encoding normalization (%7E to ~), removal of unnecessary query parameters, and path normalization (/a/../b to /b).
URL normalization is a core technology for URL shortening services. By normalizing user-submitted URLs before storing them in the database, you prevent multiple shortened URLs from being generated for the same page. If "https://example.com/page" and "https://example.com/page/" are treated as different URLs, the same page gets two shortened URLs and click statistics become fragmented.
From an SEO perspective, poor URL normalization causes duplicate content problems. When the same content is accessible at multiple URLs, search engines struggle to determine which URL is the canonical version, and search rankings become diluted. Using the canonical tag (<link rel="canonical">) to explicitly declare the canonical URL is the most reliable countermeasure.
RFC 3986 defines URI syntax, and normalization rules are based on this specification. In practice, however, strict RFC normalization alone is insufficient - site-specific normalization rules based on web server configuration (trailing slash handling, www presence, etc.) are also necessary. Related books are also available on Amazon.