Why is scanning slow on Windows?
Sometimes Angry IP Scanner can not detect open ports and will consider them as filtered.
This problem can always be ‘fixed’ by changing some scanning preferences, like timeouts and number of scanning threads.
The cause of the problem is that Angry IP Scanner doesn’t wait for responses from the hosts to arrive long enough: if the network is congested with packets or hosts just reply slowly for any reason, the roundrip time of TCP handshake can exceed the configured port timeout (see Preferences dialog, Ports tab). The default waiting time is 3 seconds, but it is decreased automatically for each host if ping packets went through quickly enough and timeout adaptation is enabled (see the corresponding check box).
Rate limiting on Windows
However, especially on Windows platforms, the problem can also be caused by TCP connection rate limiting.
Starting from Windows XP SP2 and on through Vista SP1, Microsoft has crippled down consumer versions of Windows, in order to limit the possibilities of insecure Windows machines to act as hosts for Internet attacks executed by worms and trojans. Unfortunately, these changes also made non-server editions of Windows a lot less capable for doing network administrations tasks, such as scanning.
Windows implementation of TCP connection attempt rate limiting limits the number of simultaneous connection attempts to 10. You can check if you reach this limit by examining the Event Log after scanning: look for the Event ID 4226, which corresponds to this problem.
For scanning purposes, that means you can have at most this number of scanning threads if you want to get reliable results. The number of scanning threads affects the maximum number of hosts scanned simultaneously and therefore the maximum number of connections made at each moment.
This limitation has been removed in Vista SP2 and later releases (Server 2008 SP2 and Windows 7).
See information about patching Vista. The site is about increasing the performance of BitTorrent downloads, but the same patch will dramatically improve scanning speed as well - just don’t forget to increase the number of threads in Angry IP Scanner’s preferences after applying the patch.
Windows 7 and beyond
This limitation is removed in Vista SP2, so no patching is required.