Introduction

Your Web server thinks that the HTTP data stream sent by the client (e.g. your Web browser or our CheckUpDown robot) was correct, but simply can not provide the access to the resource specified by your URL. This is equivalent to the 'return to sender - address unknown' response for conventional postal mail services.

This error is easily shown in a Web browser if try a URL with valid domain name but invalid page e.g. http://www.ibm.com/aaaaaaaaa.html.

404 errors in the HTTP cycle

Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle:

• Obtain an IP address from the IP name of your site (your site URL without the leading 'http://'). This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
• Open an IP socket connection to that IP address.
• Write an HTTP data stream through that socket.
• Receive an HTTP data stream back from your Web server in response. This data stream contains status codes whose values are determined by the HTTP protocol. Parse this data stream for status codes and other useful information.

This error occurs in the final step above when the client receives an HTTP status code that it recognises as '404'.

Resolving 404 errors - general

For top level URLs (such as www.isp.com), the first possibility is that the request for your site URL has been directed to a Web server that thinks it never had any pages for your Web site. This is possible if DNS entries are fundamentally corrupt, or if your Web server has corrupt internal records. The second possibility is that the Web server once hosted the Web site, but now no longer does so and can not or will not provide a redirection to another computer which now hosts the site. If your site is completely dead - now effectively nowhere to be found on the Internet - then the 404 message makes sense. However if your site has recently moved, then an 404 message may also be triggered. This is also a DNS issue, because the old Web server should no longer be accessed at all - as soon as global DNS entries are updated, only your new Web server should be accessed.

For low-level URLs (such as www.isp.com/products/list.html), this error can indicate a broken link. You can see this easily by trying the URL in a Web browser. Most browsers give a very clear '404 - Not Found' message.

Provided that your Web site is still to be found somewhere on the Internet, 404 errors should be rare. For top level URLs, they typically occur only when there is some change to how your site is hosted and accessed, and even these typically disappear within a week or two once the Internet catches up with the changes you have made. For low-level URLs, the solution is almost always to fix your Web pages so that the broken hypertext link is corrected.

Resolving 404 errors - CheckUpDown

Persistent 404 errors on your CheckUpDown account indicate a fundamental problem that may not be easy to resolve. If you do see lots of 404 errors, then please contact us (email preferred) so we can help you to sort them out. Unfortunately this may take some time, because we may have to liaise with your ISP and the vendor of your Web server software to agree the exact reason for the error.