Use PPTP if your enterprise doesn't have a CA to issue computer certificates and if you need compatibility with VPN client computers running legacy Windows platforms.
Use L2TP if your VPN connections require data integrity and non-reputability and not just the data confidentiality that PPTP provides.
Use PPTP if your VPN clients may be located behind NATs as most NATs can be configured to work with this protocol but not with L2TP.
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Mitch Tulloch is President of MTIT Enterprises, an IT content development company based in Winnipeg, Canada. Prior to starting his own company in 1998, Mitch worked as a Microsoft Certified Trainer (MCT) for Productivity Point International. Mitch is a widely recognized expert on Windows administration, networking and security and has written 14 books and over a hundred articles on various topics. He has been repeatedly awarded Most Valuable Professional (MVP) status by Microsoft for his outstanding contributions in supporting users who deploy Microsoft platforms, products and solutions. Mitch is also a professor at Jones International University (JIU) where he teaches graduate-level courses in Information Security Management (ISM) for their Masters of Business Administration (MBA) program. For more information see http://www.mtit.com.