301, 302, 200, 206, 304, 404 and other HTTP status introduced (reprint)

The article from the online reprint, thanks to his hard pay!

If a request is made to your server to display a webpage on your site, your server returns an HTTP status code in response to the request.

Some of the common status codes are:

200-Server successfully returned to Web page
404-The requested page does not exist
503-The server is temporarily unavailable
The following provides a complete list of HTTP status codes. Click on the link to learn more. You can also access the information about the HTTP status code in the pages for details.

1XX (Temporary response)
A status code that represents a temporary response and requires the requestor to perform an action before continuing.

Code description
100 (continued) The requesting person shall continue to make the request. The return of this code by the server means that the server has received the first part of the request and is now waiting to receive the remainder.
101 (switching protocol) The requestor has asked the server to switch protocols and the server has confirmed and is ready to switch.

2XX (Success)

The status code used to indicate that the server has successfully processed the request.

Code description
200 (success) The server has successfully processed the request. Typically, this indicates that the server provided the requested Web page. If your robots.txt file is displayed for this status, this means that Googlebot has successfully retrieved the file.
201 (created) The request was successful and the server has created a new resource.
202 (accepted) the server has accepted the request, but it has not been processed.
203 (non-authoritative information) the server successfully processed the request, but returned information that might have come from another source.
204 (no content) the server successfully processed the request, but did not return any content.
205 (reset content) the server successfully processed the request, but did not return any content. Unlike the 204 response, this response requires the requestor to reset the document view (for example, to clear the form contents to enter new content).
206 (partial) The server successfully processed a partial GET request.

3XX (redirected)
To complete the request, you need to proceed further. Typically, these status codes are always redirected. Google recommends that you use less than 5 redirects per request. You can use the Webmaster tools to see if Googlebot is having problems crawling your redirected pages. The crawl error page under diagnostics lists the URLs that Googlebot cannot crawl due to redirection errors.

Code description
300 (multiple choices) the server can perform a variety of actions depending on the request. The server can select an action based on the requestor (user agent) or provide a list of actions for the requestor to select.
301 (permanently moved) The requested page has been permanently moved to the new location. When the server returns this response (as a response to a GET or HEAD request), the requestor is automatically forwarded to the new location. You should use this code to notify Googlebot that a Web page or Web site has been permanently moved to a new location.
302 (Temporary move) The server is currently responding to requests from a Web page in a different location, but the requestor should continue to use the original location for future requests. This code, like the 301 code that responds to the GET and HEAD requests, automatically transfers the requestor to a different location. However, since Googlebot will continue to crawl the original location and index it, you should not use this code to notify Googlebot that a page or site has been moved.
303 (See other locations) The server returns this code when the requestor should make a separate GET request for a different location to retrieve the response. For all requests except the HEAD request, the server automatically goes to a different location.
304 (not modified) The requested webpage has not been modified since the last request. When the server returns this response, the Web page content is not returned.

If the page has not changed since the requestor last requested it, you should configure the server to return this response (known as the If-modified-since HTTP header). Because the server can tell Googlebot that the page has not changed since the last crawl, it saves bandwidth and overhead

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305 (using a proxy) the requestor can only use the proxy to access the requested Web page. If the server returns this response, the server also indicates which proxy the requestor should use.
307 (Temporary redirect) The server is currently responding to requests from a Web page in a different location, but the requestor should continue to use the original location for future requests. This code, like the 301 code that responds to the GET and HEAD requests, automatically transfers the requestor to a different location. However, since Googlebot will continue to crawl the original location and index it, you should not use this code to notify Googlebot that a page or site has been moved.

4xx (Request error)
These status codes indicate that the request may have failed and that the server has been prevented from processing the request.

Code description
400 (Error request) The server does not understand the syntax of the request.
401 (unauthorized) request authentication is required. After logging in, the server may return this response to the page.
403 (disabled) the server denied the request. If this status code appears when Googlebot tries to crawl a valid webpage on your site (you can see this status code on the Web crawl page in the Google Webmaster Tools diagnostics), this could be a denial of access by your server or host to Googlebot.
404 (not found) The requested Web page was not found by the server. For example, if the request is for a webpage that does not exist on the server, the server typically returns this code.

If you do not have a robots.txt file on your site and you find this status on the robots.txt page of the Google Webmaster Tools Diagnostics tab, this is the correct state. However, if you have a robots.txt file and you find this status, this means that your robots.txt file may be named incorrectly or in the wrong location. (the file should be located on the top-level domain name and should be named Robots.txt).

If you find this status on the URL Googlebot trying to crawl (on the HTTP error page of the Diagnostics tab), this means that Googlebot may be tracking an invalid link (old link or incorrect input link) on another page.

405 (method Disabled) Disables the method specified in the request.
406 (not accepted) The requested content attribute cannot be used in response to the requested Web page.
407 (requires proxy authorization) This status code is similar to 401 (unauthorized), but specifies that the requestor should use the proxy for authorization. If the server returns this response, the server also indicates which proxy the requestor should use.
408 (Request timed out) The server timed out while waiting for the request.
409 (conflict) The server has a conflict when it finishes the request. The server must contain information about the conflicts that occurred in the response. The server may return this code in response to a PUT request that conflicts with the previous request, and will provide a list of differences for two requests.
410 (Deleted) If the requested resource has been permanently deleted, the server returns this response. This code is similar to the 404 (not Found) code, but sometimes replaces the 404 code in cases where resources have previously existed but are no longer present. If the resource has been permanently deleted, you should use 301 code to specify a new location for the resource.
411 (requires valid length) The server does not accept requests that contain invalid content-Length header fields.
412 (precondition not met) the server does not meet one of the prerequisites set by the requestor in the request.
413 (Request entity is too large) the server cannot process the request because the request entity is too large to exceed the processing power of the server.
414 (The requested URI is too long) The requested URI (usually the URL) is too long for the server to process.
415 (Unsupported media type) The requested format is not supported by the requested page.
416 (Request scope does not meet the requirements) if the request is made for an invalid range of pages, the server returns this status code.
417 (unmet expectations) the server does not meet the requirements for the expected Request header field.

5XX (server error)
These status codes indicate that an internal error occurred while the server was trying to process the request. These errors may be the error of the server itself, not the request.

Code description
500 (server internal error) the server encountered an error and could not complete the request.
501 (not yet implemented) the server does not have the capability to complete the request. For example, the server may return this code when the server does not recognize the request method.
502 (Error Gateway) The server received an invalid response from the upstream server as a gateway or proxy.
503 (Service Unavailable) the server is not currently available (due to overloading or downtime maintenance). Usually, this is just a temporary state.
504 (Gateway Timeout) The server acts as a gateway or proxy and does not receive requests from the upstream server in a timely manner.
505 (HTTP version not supported)

thanks to the original article blogger, learned a lot of things!

301, 302, 200, 206, 304, 404 and other HTTP status introduced (reprint)