IIS Request Restrictions on Windows Client OS

IIS on Windows XP:

• Can quickly exceed it's connection limit.
• Has a maximum of 10 concurrent connections before an HTTP 403.9 error message is returned. HTTP 403.9 returns Access Forbidden: Too many users are connected.

The major limitation on Windows Client OS for IIS 7.x and later is the number of concurrent requests IIS can execute. This limitation is fundamentally different from the Windows XP connection limit. The HTTP 403.9 error is unlikely to occur when concurrent requests are limited. Requests that cannot be handled because the concurrent request limit is reached are queued.

If the request queue overflows, a 500 Internal Server Error is returned. The request queue can handle thousands of requests by default. Too many concurrent requests doesn't generate an error, but generally slows response time.

Windows Server OS doesn't have these request restrictions.

The maximum concurrent requests IIS 7.x and later allows on Windows Client OS SKUs:

Windows 10 – IIS 10.0 Concurrent Requests Limit

Edition	limit
Home	3
Professional	10
Enterprise	10

Windows 10 IoT – IIS 10.0 Concurrent Requests Limit

Edition	limit
Enterprise 2016	10
Enterprise 2019	10

Windows 8.1 – IIS 8.5 Concurrent Requests Limit

Edition	limit
Basic	3
Professional	10
Enterprise	10

Windows 8 – IIS 8 Concurrent Requests Limit

Edition	limit
Basic	3
Professional	10
Enterprise	10

Windows 7 - IIS 7.5 Concurrent Requests Limit

Edition	limit
Home Starter	1
Basic	1
Home Premium	3
Professional	10
Ultimate	10
Enterprise	10

Windows Vista - IIS 7.0 Concurrent Requests Limit

Edition	limit
Home Basic	3
Home Premium	3
Professional	10
Ultimate	10
Enterprise	10