HTTP 411 Length Required vs 511 Network Authentication Required
HTTP 411 (Length Required) is a 4xx Client Error response, while 511 (Network Authentication Required) is a 5xx Server Error response. 411 indicates that the server refuses to accept the request without a Content-Length header. In contrast, 511 means that the client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi).
Description
The server refuses to accept the request without a Content-Length header.
When You See It
When sending a request body without specifying Content-Length.
How to Fix
Include a Content-Length header in your request.
Description
The client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi).
When You See It
When connecting to WiFi that requires login (airport, hotel, coffee shop).
How to Fix
Open a browser and complete the captive portal login process.
Key Differences
411 is a 4xx Client Error response, while 511 is a 5xx Server Error response.
HTTP 411: The server refuses to accept the request without a Content-Length header.
HTTP 511: The client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi).
You encounter 411 when when sending a request body without specifying Content-Length.
You encounter 511 when when connecting to WiFi that requires login (airport, hotel, coffee shop).
When to Use Which
For 411 (Length Required): Include a Content-Length header in your request. For 511 (Network Authentication Required): Open a browser and complete the captive portal login process.