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After entering the URL in the browser and pressing Enter, the browser sends a request to the server, and the server sends back a response describing the status of the resource.
Request Headers will bring information such as User-Agent, target path, and supported compression methods.
Response Headers will describe the return file and server processing results, among which the Status Code is most commonly used by SEO workers.
2XX usually means success, 3XX means you need to go to another location, 4XX usually means there is a problem with the request or the resource itself, and 5XX usually means a server-side error occurred.
After understanding this basic process, 404, 301, HTTPS, cache and crawler are no longer separate terms, but different signals in the life cycle of the same web page request.
Links mentioned in the original post
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