Paste or upload sitemap XML and check its structure: valid XML, the right root element, and complete, unique location entries.
A sitemap is an XML file with a strict structure: a urlset root holding one url entry per page, or a sitemapindex root holding one sitemap entry per child sitemap file, with a loc element giving each address. This validator parses the pasted or uploaded XML, confirms it is well-formed, checks the root element, and then checks every entry for a present, absolute, well-formed loc value, duplicate addresses, and the protocol limit of 50,000 entries per file. It also checks lastmod values that are present. It performs structural checks only: it does not fetch any listed URL, so it cannot confirm that a page exists or that a search engine will accept the file.
Does this validator fetch a sitemap from a URL? No. Nothing is fetched from any website. Paste the XML or open a local sitemap file, and the checks run on that content in the browser.
What is the difference between a urlset and a sitemap index? A urlset lists page URLs directly. A sitemap index lists other sitemap files, each in a sitemap entry, and is used when a site has more URLs than one file can hold. Both use loc entries, which this validator checks either way.
How many URLs can one sitemap file contain? The sitemap protocol allows up to 50,000 URLs per file. This validator flags a file over that limit, along with missing, malformed, or duplicate loc entries.