How many simultaneous sockets can I have open?

There is no fixed connection or socket limit on any modern version of Windows. The limit depends on the I/O strategy you use, the amount of memory in the system, and your program’s network usage pattern:

The I/O Strategy Factor: As the above-linked article points out, there are many possible I/O strategies with Winsock. This is because they have different strengths and weaknesses, one of which is how well the strategy fares in high connection count situations. If you have to get into the thousands of connections, you want to use overlapped I/O, the only strategy that reliably allows you to achieve such high connection counts. Other strategies ? asynchronous notification, select(), thread-per-socket... ? will hit some other performance wall before the network stack itself starts running out of resources. Some take too much CPU, others require lots of context switches, others use inefficient notification mechanisms.

The Memory Factor: According to Microsoft, all modern versions of Windows allocate sockets out of the non-paged memory pool. (That is, memory that cannot be swapped to the page file by the virtual memory subsystem.) The size of this pool is necessarily fixed, and is dependent on the amount of physical memory in the system. On Intel x86 machines, the non-paged memory pool stops growing at 1/8 the size of physical memory, with a hard maximum of 128 megabytes for Windows NT 4.0, and 256 megabytes for Windows 2000. You thus hit the maxium with 1 or 2 GB of RAM, respectively. (I do not have any information on whether these limits have increased in newer versions of Windows, or if something different happens on 64-bit CPUs. If you do, email me.)

The “Busy-ness” Factor: The amount of data associated with each socket varies depending on how that socket’s used, but the minimum size is around 2 KB. Overlapped I/O buffers also eat into the non-paged pool, in blocks of 4 KB. (4 KB is the x86 memory management unit’s page size.) Thus a simplistic application that’s regularly sending and receiving on a socket will tie up at least 10 KB of non-pageable memory. Assuming that simple case of 10 KB of data per connection, the theoretical maximum number of sockets on NT 4.0 is about 12,800, and on Win2K 25,600.

I have seen reports of a 64 MB Windows NT 4.0 machine hitting the wall at 1,500 connections, a 128 MB machine at around 4,000 connections, and a 192 MB machine maxing out at 4,700 connections. It would appear that on these machines, each connection is using between 4 KB and 6 KB. The discrepancy between these numbers and the 10 KB number above is probably due to the fact that in these servers, not all connections were sending and receiving all the time. The idle connections will only be using about 2 KB each.

So, adjusting our “average” size down to 6 KB per socket, NT 4.0 could handle about 22,000 sockets and Win2K about 44,000 sockets. The largest value I’ve seen reported is 16,000 sockets on Windows NT 4.0. This lower actual value is probably partially due to the fact that the entire non-paged memory pool isn’t available to a single program. Other running programs (such as core OS services) will be competing with yours for space in the non-paged memory pool.

출처 : http://tangentsoft.net/wskfaq/advanced.html