The proper HTTP status for “too many connections?”

because the number of connections is ip bound in most browsers, you can spread your content across multiple ips/domains to enhance performance. you can find a more detailed explanation at yahoo. there are also some more performance tips for your frontend performance.

a good tool for messauring frontend performance in firefox is yslow. it does not only messaure performance, but also gives hints about how to improve your performance.

I would imagine that if you checked most of those connections will be on port 80 and in a TIME_WAIT state.

TIME_WAIT is a state where connections could conceivably be re-used without making a new TCP between servers, so it can help reduce load.

So basically your router is saving a tcp socket for later use to the same site.