Most of you know the 404 error, which means the server can't find the page you're looking for.

What you may not know is that there are errors other than the 404. There's a whole set of error messages. You're probably most familiar with the 400 and 500 level errors. These are the most common ones to pop up in a browser window.

The 404, and all of its numeric partners, are what are known as Server Response Codes (SRC). They're created when the server itself sends back a message that all is okay, or that something has gone wrong. Most of these you'll never see as they act silently. Meaning, the response is given that all is okay and the process continues without you ever seeing the response given. Those SRCs are numbers found mostly within 100 through 399.

The messages you see are generally those mostly within the 400 to 599 range.

In the descriptions below, the "header" is mostly the reason for the error.

Headers

A header is text that is used to transfer information between clients and servers. You will never see the headers unless you get deeper into programming on the server itself. They're created by the machines that move information around on the Web. There are basically four types of headers:

1. General: This is information related to the client, the server, or the protocol (like http or ftp).
2. Entity: This is information about the actual data being transferred.
3. Request: This sets standards regarding acceptable formats and parameters.
4. Response: This contains information about the server sending the response.

Every time a server is contacted by a user, a "handshake" occurs. This is where the two machines communicate to each other and make sure they can connect to effect the transfer you requested. These headers are used to provide all the information required to have a successful handshake.

The codes below are created as a result of that handshake.

The Server Response Codes

Here's how it all breaks down:
100-199	SRCs provide confirmation that a request was received and is being processed (silent).
100	This is good. The request was completed and the process can move along.
101	Request to switch protocols (like from HTTP to FTP) was accepted.
200-299	SRCs report that requests were performed sucessfully. (silent)
200	It simply means all is okay. What the client requested is available.
201	This means a new address was successfully created through a CGI or posting form data.
202	The client's request was accepted, although not yet acted upon.
203	The accepted information in the entity header is not from the original server, but from a third party.
204	There is no content in the requested click. If you click on an image map section not attached to a page. This allows the server to just sit there waiting for another click rather than throwing an error.
205	This allows the server to reset the content returned by a CGI.
206	Only partial content is being returned for some reason.
300-399	Request was not performed, but a redirection is occurring (usually silent).
300	The requested address refers to more than one entity. Depending on how the server is configured, you get an error or a choice of which page you want.
301	Page has been moved permanently, and the new URL is available. You should be sent there by the server.
302	Page has been moved temporarily, and the new URL is available. You should be sent there by the server.
303	This is a "see other" SRC. Data is somewhere else and the GET method is used to retrieve it.
304	This is a "Not Modified" SRC. If the header in the request asks "If Modified Since", this will return how long it's been since the page was updated.
305	This tells the server the requested document must be accessed by using the proxy in the Location header (i.e., ftp, http).
400-499	Request is incomplete for some reason.
400	There is a syntax error in the request. It is denied.
401	The header in your request did not contain the correct authorization codes. You cannot see what you requested.
402	Payment is required. It's not in use yet.
403	You are forbidden to see the document you requested. It can also mean that the server doesn't have the ability to show you what you want to see.
404	Document not found. The page you want is not on the server nor has it ever been on the server. You may have misspelled the title or used an incorrect capitalization pattern in the URL.
405	The method you are using to access the file is not allowed.
406	The page you are requesting exists but you cannot see it because your own system doesn't understand the format the page is configured for.
407	The request must be authorized before it can take place.
408	The request timed out. For some reason the server took too much time processing your request. Net congestion is the most likely reason.
409	Conflict. Too many people wanted the same file at the same time. Server overload. Try again.
410	The page used to be there, but now it's gone.
411	Your request is missing a Content-Length header.
412	The page you requested has some sort of precondition set up. That means that if something is a certain way, you can have the page. If you get a 412, that condition was not met.
413	Too big. What you requested is just too big to process.
414	The URL you entered is too long.
415	The page is an unsupported media type, like a proprietary file made specifically for a certain program.
500-599	Errors have occurred in the server itself.
501	What you requested of the server cannot be done by the server.
502	Your server has received errors from the server you are trying to reach. The "Bad Gateway" error.
503	The format or service you are requesting is temporarily unavailable.
504	The gateway has timed out. Similar to 408 error except that the time-out occurred specifically at the gateway of the server.
505	The HTTP protocol you are asking for is not supported.