Sitemap is a XML file which contains a full list of your website URLs. It is used to suggest URLs for search engines to crawl and index. It can greatly help search engine robots index your website quickly and deeply. It is roughly an opposite of robots.txt.
You can create a sitemap.xml by various free and paid service, or you can write it with proper way (read about how write a sitemap).
Keep these things in mind:
1) Sitemap must be less than 10 MB (10,485,760 bytes), and contain a maximum 50,000 URL. if you have more URLs or if your file is larger than 10 MB is size, then create multiple sitemap files, and use a sitemap index file.
2) Put your sitemap in website root directory, and add the URL of your sitemap in robots.txt. For example: Sitemap: /sitemap.xml
3) sitemap.xml can be compressed using grip for faster loading.
A broken link is an inaccessible link URL. A higher rate of broken links have a negative effect on search engine ranking due to reduced link equity. It also has a bad impact on user experience. There are several reasons for broken links:
1) An incorrect link entered by you.
2) The destination website removed the linked webpage given by you. (A common 404 error).
3) The destination website is irreversibly moved or no longer exists. (domain changed or site blocked or site offline).
4) User may be behind a firewall or security mechanism that is blocking the access to the destination website.
5) You have provided a link to a site that is blocked by firewall for outside access.
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