Lifepal has an interesting article about HTTP Response Status Codes. The HTTP request response cycle begins with DNS Lookup and ends with the browser parsing the response after studying the Response headers.
Response headers are a way by which the browser understands what happened when the request was sent to the server. In a way, for the browser its a way to identify answer to many questions like Did the server process the request successfully? Do i need to check in some other location? Is the content being sent in ranges? Is the server available at all or not?
In general , HTTP Status Codes have following meanings.
1xx ( All HTTP Response Status codes beginning with 1) : Status codes that indicate a provisional response and require the requestor to take action to continue.
2xx ( All HTTP Response Status codes beginning with 2) : Status codes that indicate that the server successfully processed the request.
3xx ( All HTTP Response Status codes beginning with 3) : Further action is needed to fulfill the request. Often, these status codes are used for redirection.
4xx ( All HTTP Response Status codes beginning with 4) : These status codes indicate that there was likely an error in the request which prevented the server from being able to process it.
5xx ( All HTTP Response Status codes beginning with 5) : These status codes indicate that the server had an internal error when trying to process the request. These errors tend to be with the server itself, not with the request.
You can read the entire article on What are HTTP response codes here
Also read: How does a DNS work?
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