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When a request is made to your server for a page on your site (for instance, when a user accesses your page in a browser or when a search engine crawls the page), your server returns an HTTP status code in response to the request.
This status code provides information about your site and the requested page.
HTTP Status Code List
Here are the most common HTTP status codes that you need to know about it to ensure your site and pages are optimized for search engines and SEO:
200 OK A good URL should always return 200. This means that the browser found the URL and the server returned a good content page.
301 Moved Permanently To move a page (or entire website), always use 301 redirects for good SEO. This tells search engines to follow the new website, transferring all the backlinks received by the old page to the new page. It is not uncommon to find a 301 redirect code when checking the URL of a site that is using their .htaccess file to redirect their www to non-www (or vice-versa).
403 Forbidden The server is refusing the request. If you see that Googlebot received this status code when trying to crawl valid pages of your site (you can see this on the Crawl Errors page under Health in Google Webmaster Tools), its possible that your server or host is blocking Googlebots access. This obviously has a huge impact on your SEO and needs to be fixed quickly.
404 Not Found Cannot find your website/page. You need to find out why and fix it.
503 The Server is Temporarily Unavailable You obviously need to investigate whats wrong with your server or host.
Check your website response with our Get HTTP Headers tool.