Half-open limit fix is a program designed to change the maximum number of concurrent half-open outbound TCP connections (connection attempts) in the Windows system tcpip.sys file.

Microsoft first introduced this limit in Windows XP SP2 and it is present in later versions, which purpose is to try to slow the spread of virus and malware from system to system, and also to reduce the impact of infected systems participating in DoS (Denial of Service) attacks.

Microsoft's limiter makes it impossible for Windows systems to have more than 10 concurrent half-open outbound connections; after 10, new connection attempts are put in a queue and forced to wait. Thus, the speed of connection to other PCs is limited.Therefore, the speed of connection to other computers is actually limited.

P2P (peer-to-peer) programs like µTorrent, BitComet, eMule, P2P TV, etc.,are generally the programs mostly affected. Since bittorrent apps tend to use up all 10 of the half-open connections, other Internet activity—especiallythe loading of web pages—can become extremely slow. The delay before opening a new page can take over ten seconds: this delay occurs regardless of your Internet connection speed.

Half-open limit fix remedies this problem by increasing the maximum limit of half-open connections. For most Internet users, changing the limit to 100 will be more than sufficient.

Version 4.1 (June 20, 2009)
- Support for Windows 7 beta starting from build 7100 is removed (limit is physically present in the tcpip.sys file but has no effect)
- Added Japanese interface (translated by wwwcfe)
- Added Hebrew interface (translated by s_h)
- Added Chinese (Traditional) interface (translated by Danfong Hsieh)
June 24, 2009:
- English in site and in program corrected by Gerard
September 6, 2009:
- Added Polish interface (translated by Tomasz “Sajgon” Wnukowski)
- Added Estonian interface (translated by Geko)
- Added Arabic interface (translated by Mohamed El-Mekawy)