If you have ever browsed the internet or worked with websites and applications, you have probably encountered errors such as 404 Not Found, 403 Forbidden, or 500 Internal Server Error. These messages are known as HTTP status codes. They are sent by a web server to tell a browser, application, or API what happened after a request was received. HTTP status codes are divided into different categories: 2xx – Successful responses 3xx – Redirections 4xx – Client errors 5xx – Server errors Understanding these codes is essential for web developers, software engineers, website owners, and anyone who wants to understand how websites and APIs work. What Are HTTP Status Codes? An HTTP status code is a three-digit number returned by a server after receiving an HTTP request. For example, when you enter a website address into your browser, the process generally looks like this: Browser → Request → Server The server processes the request and sends back a response. If everything works correctly, the server might return: 200 OK If the requested page does not exist, it might return: 404 Not Found Therefore, HTTP status codes provide information about the result of a request. 2xx – Successful Responses HTTP status codes beginning with 2 indicate that the request was successfully received, understood, and processed. 200 OK 200 OK is one of the most common HTTP status codes. It means the request was successfully completed. For example, when you visit a working webpage, the server will typically return a 200 OK response. 201 Created 201 Created is used when a new resource has been successfully created. For example, an API might return 201 after successfully creating a new user account. 204 No Content 204 No Content means the request was successfully processed, but the server does not need to return any content. This status code is commonly used with APIs. 3xx – Redirection Responses HTTP status codes beginning with 3 generally indicate that the requested resource has been moved or that the client needs to take another action. 301 Moved Permanently 301 Moved Permanently means that a URL has permanently moved to another location. For example: Old URL → New URL 301 redirects are especially important for SEO when a website changes its URLs because they help redirect visitors and search engines to the new location. 302 Found 302 Found is commonly used for a temporary redirect. For example, a website might temporarily redirect visitors from one page to another while keeping the original URL as the main address. 304 Not Modified 304 Not Modified tells the browser that a resource has not changed since the last time it was requested. The browser can therefore use the cached version instead of downloading the resource again. 4xx – Client Errors HTTP status codes beginning with 4 usually indicate that there is a problem with the request made by the client. These are commonly referred to as HTTP client errors. 400 Bad Request 400 Bad Request means the server could not understand or process the request. Possible causes include: Invalid data Incorrect request format Missing parameters Invalid API requests 401 Unauthorized 401 Unauthorized is generally related to authentication. For example, a user may try to access an API or protected page without providing valid authentication credentials. 403 Forbidden 403 Forbidden means the server understood the request but refuses to allow access. For example, a user without the required permissions may attempt to access an administrator dashboard. The server can respond with: 403 Forbidden 404 Not Found 404 Not Found is one of the most recognizable HTTP errors on the web. It means the server could not find the requested page or resource. For example: example.com/unknown-page If that page does not exist, the server may return: 404 Not Found 429 Too Many Requests 429 Too Many Requests occurs when a client sends too many requests within a specific period. It is commonly used for: API rate limiting Preventing abuse Protecting servers from excessive traffic 5xx – Server Errors HTTP status codes beginning with 5 usually indicate that the server encountered a problem while processing a valid request. 500 Internal Server Error 500 Internal Server Error is one of the most common server errors. It means that something unexpected went wrong on the server. Possible causes include: Application bugs Database problems Server configuration issues Plugin errors Programming errors 502 Bad Gateway 502 Bad Gateway usually occurs when a server or proxy receives an invalid response from another server. It can happen on websites that use: Reverse proxies Load balancers Multiple servers 503 Service Unavailable 503 Service Unavailable means the server is temporarily unable to handle the request. Possible causes include: Server overload Scheduled maintenance Temporary service failures 504 Gateway Timeout 504 Gateway Timeout occurs when a server or gateway waits too long for a response from another server. In simple terms, one server was waiting for another server, but the response did not arrive in time. HTTP Status Codes Quick Reference CodeMeaningCategory200OKSuccess201CreatedSuccess204No ContentSuccess301Moved PermanentlyRedirect302FoundRedirect304Not ModifiedCache400Bad RequestClient Error401UnauthorizedClient Error403ForbiddenClient Error404Not FoundClient Error429Too Many RequestsClient Error500Internal Server ErrorServer Error502Bad GatewayServer Error503Service UnavailableServer Error504Gateway TimeoutServer Error HTTP Client Errors vs Server Errors One of the most important differences to understand is between 4xx and 5xx errors. 4xx Errors These generally indicate a problem with the client's request. For example: 404 – The requested page cannot be found. 5xx Errors These generally indicate a problem on the server side. For example: 500 – The server encountered an unexpected error. A simple way to remember it is: 4xx = Client-side problem 5xx = Server-side problem Why Are HTTP Status Codes Important for Developers? HTTP status codes help developers understand how their applications and APIs are responding. For example: User successfully created → 201 Request completed successfully → 200 Authentication required → 401 Access denied → 403 Resource doesn't exist → 404 Server encountered an error → 500 Using the correct status codes also makes APIs easier for other developers to understand and debug. HTTP Errors and SEO HTTP status codes are also important for SEO and website management. 200 Status A 200 response indicates that a webpage is available and working normally. 301 Redirect A 301 redirect is useful when permanently moving a page to a new URL. 404 Error A 404 indicates that the requested page does not exist. 5xx Errors Frequent server errors can create problems for visitors and search engine crawlers. Website owners should therefore regularly monitor their websites for broken pages, redirects, and server errors. How to Find HTTP Errors Developers can use several tools to identify HTTP status codes and errors. Common options include: Browser Developer Tools Google Search Console Postman cURL Server Logs Website Monitoring Tools In most browsers, you can open Developer Tools → Network to see the status code returned for each request. Final Thoughts HTTP status codes are an essential part of web development and API development. Understanding 2xx, 3xx, 4xx, and 5xx responses helps developers identify whether a request was successful, redirected, rejected, or affected by a server problem. If you're a beginner, start by learning the most common codes such as 200, 301, 302, 400, 401, 403, 404, 500, 502, 503, and 504. Once you understand what these codes mean, troubleshooting websites, building APIs, and maintaining web applications becomes much easier. https://samoskill.com/understanding-client-server-architecture-and-how-the-internet-works/ https://samoskill.com/understanding-web-architecture-front-end-vs-back-end-development/ https://samoskill.com/native-mobile-development-vs-cross-platform-development/
