Two related questions:
One interesting way to get around the X connections per server limit is to map static resources like scripts and images to their own domains... img.foo.com or js.foo.com.
I have only read about this - not actually tried or tested. So please let me know if this doesn't work.
Most HTTP servers have little ability to restrict the number of connections from a single host, other than to ban the IP. In general, this isn't a good idea, as many users are behind a proxy or a NAT router, which would allow for multiple connections to come from the same IP address.
On the client side, you can artificially increase this amount by requesting resources from multiple domains. You can setup www1, www2, etc.. alias which all point to your same web server. Then mix up where the static content is being pulled from. This will incur a small overhead the first time due to extra DNS resolution.
The limitiation is usually the web server. It's common that a web server only allows two concurrent downloads per user.
Active scripting engines like ASP.NET only executes one request at a time per user. Requests for static files are not handles by the scripting engine, so you can still get for example an image while getting an aspx file.
Pages often have content from different servers, like traffic measuring scripts and such. As the download limit is per server you can typically download two files at a time from each server.
I know at least in Firefox, this value is configurable (
network.http.pipelining.maxrequests), so I doubt you'll get a definitive answer on this one. The default is 4, however.
What are you attempting to accomplish?
There is nothing in HTTP that limits the number of sessions.
However, there are configuration items in FF for one, that specifically set how many total sessions and how many sessions to a single server are allowed. Other browsers may also have this feature.
In addition, a server can limit how many sessions come in totally and from each client IP address.
So the correct answer is:
1/ The number is sessions is limited to the minimum of that imposed by the client (browser) and server.
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