Common HTTP response status codes and meanings

Communication over HTTP, the Hypertext transfer protocol is communication between web server and clients that involves transferring packets of data from one to the other in stateless form.

These packets of data are formatted and are called HTTP messages. It could be a Request message or a Response message. There is also the notion of Inbound and Outbound message.

When an HTTP message flows to the server (a request message), it is termed an Inbound flow. When an HTTP message flows back to the client from the server, it is termed an Outbound flow.

This Inbound and Outbound flow of data is also considered by Hosting providers, such as AWS, Google Cloud Platforms, Azure, etc., as they charge for data flow.

HTTP Response Status Code

When applications communicate over the web, there need to be standardized status codes or signal to indicate HTTP request or operation status. HTTP status codes are 3-digit codes that indicate the status of HTTP operations.

An HTTP request or operation can be in many states. It could be pending, it could error out, it could be in a success state, it could fail, and so many others. This need led to what we call today, HTTP Response Status Codes.

HTTP Response Status Code is a three-digit code that indicates the status of an HTTP request-response operation.

The nature of HTTP messages

The HTTP message is a simple but formatted block of data, either a request message from a web client or an enclosing request-response message from a web server.

HTTP messages consist of three parts:

The Start Line of an HTTP message either says what to do (for a request message) or explains what happened (for a response message).

The second part, a block of headers, describes the content of the message. Information such as the content length, the content type, the age of the content and many others are found within the headers' block.

Both the request and response message follow the same with three parts, as stated above. The syntax of both messages differs only in the start line.

Request Message Format

Below is the format for an HTTP request message. The start line, which is the first line contains the Request Method, the Request URL and the HTTP version.

<Method> <Request-url> <Version>
<Headers>

<Entity-body>

Response Message Format

Below is the format for an HTTP response message. The start line contains the HTTP version, the status code and the reason phrase. The response message format differs from the request message on the start line.

<Version> <status-code> <reason-phrase>
<Headers>

<Entity-body>

As you can see, The response status code, the second entry in the start line of an HTTP response message, is of interest. The third-entry reason-phrase is a human-readable version of the status-code.

Classification of HTTP Response Status Codes

HTTP response status codes can be divided into 5 categories based on the type of information the status code relays to the receiving client. The five HTTP response status codes categories are:

As stated earlier, HTTP status codes are 3-digit codes.

Informational HTTP Status Codes - 1xx

Informational HTTP Status Codes are status codes within the range of 100 to 199 both inclusive. They are not well conversant around the internet and were introduced by the HTTP 1.1 protocol.

Servers Implementing just the HTTP 1.0 do not support this range of status codes. Informational HTTP Status Codes are performantly intended to help reduce the burden on web servers.

According to the HTTP 1.1 specification, Implementing Servers, proxies must not send a 1xx response back to HTTP 1.0 clients as they do not support these range of headers.

HTTP Success Status Codes - 2xx

HTTP Success Status Codes are status codes within the range of 200 to 299, both inclusive. The 200 and 201 status codes are the most common within this range.

HTTP Success Status Codes indicates that the client's request was successfully received, stored, processed or retrieved with various kinds of reason phrase depending on the nature of the request especially the request method.

HTTP Redirection Status Codes - 3xx

When a resource URI changes, when the resource location changes, servers utilize the 3xx response codes to convey information to requesting clients on the new location of such a resource.

Redirection status codes are in the range of 300-399 both inclusive. 3xx response status codes tell the connected client to use alternate locations for the resources requested.

Coupled with the Location header, Servers can provide the temporal or permanent alternate URL of the requested resource to clients.

Client Error Response Status Codes - 4xx

Sometimes, Client requests can be erroneous, such as invalid entity-body, no authorization headers, authentication headers, and many others. The 4xx response codes indicate errors in the client request.

Clients can also request for a non-existing resource, hence, the famous 404 error response code.

Client Error HTTP status codes are in the range of 400-499 both inclusive. 4xx response status codes tell the connected client about the error in the request.

Server Error Response Status Codes - 5xx

Aside from client request errors, servers can encounter errors while processing client requests. The 5xx response codes indicate server-side errors that occurred while processing a client request.

Server errors can range from system failure, network errors, code or script errors, gateway errors, and many others. Troubleshooting such errors by server administrators usually involves looking up server error log files.

Server Error HTTP status codes are in the range of 500-599 both inclusive. 5xx response status codes tell the connected client about the server's error which occurred while processing the request.