Some websites are locked down so that only those on the local company or school network can access parts of the site. You will often see 403 Forbidden errors when browsing such sites from "off-campus."
Sometimes webmasters try to set up dynamic web programming features like PHP or Perl/CGI but fail to do so correctly. This can also result in 403 Forbidden errors until the web server is correctly configured.
403 Forbidden can appear in other situations at the discretion of the webmaster, so you may see it in scenarios other than these.
For a complete list of standard HTTP status codes, see the W3 Consortium's website.
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