HTTP Status Codes

1xx informational response – the request was received, continuing process

• 100 Continue
• 101 Switching Protocols
• 102 Processing (WebDAV; RFC 2518)
• 103 Early Hints (RFC 8297)

2xx successful – the request was successfully received, understood, and accepted

• 200 OK
• 201 Created
• 202 Accepted
• 203 Non-Authoritative Information (since HTTP/1.1)
• 204 No Content
• 205 Reset Content
• 206 Partial Content (RFC 7233)
• 207 Multi-Status (WebDAV; RFC 4918)
• 208 Already Reported (WebDAV; RFC 5842)
• 226 IM Used (RFC 3229)

3xx redirection – further action needs to be taken in order to complete the request

• 300 Multiple Choices
• 301 Moved Permanently
• 302 Found (Previously "Moved temporarily")
• 303 See Other (since HTTP/1.1)
• 304 Not Modified (RFC 7232)
• 305 Use Proxy (since HTTP/1.1)
• 306 Switch Proxy
• 307 Temporary Redirect (since HTTP/1.1)
• 308 Permanent Redirect (RFC 7538)

4xx client error – the request contains bad syntax or cannot be fulfilled

• 400 Bad Request
• 401 Unauthorized (RFC 7235)
• 402 Payment Required
• 403 Forbidden
• 404 Not Found
• 405 Method Not Allowed
• 406 Not Acceptable
• 407 Proxy Authentication Required (RFC 7235)
• 408 Request Timeout
• 409 Conflict
• 410 Gone
• 411 Length Required
• 412 Precondition Failed (RFC 7232)
• 413 Payload Too Large (RFC 7231)
• 414 URI Too Long (RFC 7231)
• 415 Unsupported Media Type (RFC 7231)
• 416 Range Not Satisfiable (RFC 7233)
• 417 Expectation Failed
• 418 I'm a teapot (RFC 2324, RFC 7168)
• 421 Misdirected Request (RFC 7540)
• 422 Unprocessable Entity (WebDAV; RFC 4918)
• 423 Locked (WebDAV; RFC 4918)
• 424 Failed Dependency (WebDAV; RFC 4918)
• 425 Too Early (RFC 8470)
• 426 Upgrade Required
• 428 Precondition Required (RFC 6585)
• 429 Too Many Requests (RFC 6585)
• 431 Request Header Fields Too Large (RFC 6585)
• 451 Unavailable For Legal Reasons (RFC 7725)

5xx server error – the server failed to fulfill an apparently valid request

• 500 Internal Server Error
• 501 Not Implemented
• 502 Bad Gateway
• 503 Service Unavailable
• 504 Gateway Timeout
• 505 HTTP Version Not Supported
• 506 Variant Also Negotiates (RFC 2295)
• 507 Insufficient Storage (WebDAV; RFC 4918)
• 508 Loop Detected (WebDAV; RFC 5842)
• 509 Bandwidth Limit Exceeded (Apache Web Server/cPanel)
• 510 Not Extended (RFC 2774)
• 511 Network Authentication Required (RFC 6585)
• 529 Site is overloaded
• 530 Site is frozen
• 598 (Informal convention) Network read timeout error
• 599 Network Connect Timeout Error

By Ping Now, a website/API/service uptime monitor
Data from Wikipedia