The open design of the Internet has not only opened many new opp- tunities for communications, but it has also opened many new avenues for attackers against organisations network and computing resources. This book is a critical investigation of the Internet Protocol Security (IPSec) based on combination of theoretical investigation and practical implementation, which provides an in-depth understanding of the IPSec framework. The bene?ts of IPSec were exploited while the delimiting factors cannot be ignored. Infor- tion security has become a major concern in recent times as more and more computers are being connected to the global Internet. With so much data tra- ferring over public networks, the risk of sensitive information has increased exponentially and the increase of Internet hosts continuously requires ad- tional security support. The IPSec may be used in three different security - mains: Virtual private networks, Application-level security, and Routing se- rity. It comprises of suite of protocols, which are developed to ensure that the integrity, con?dentiality and authentication of data communications over an IP network. The IPSec is predominately used in virtual private networks (VPNs). But when used in application-level security or routing security, the IPSec is not a complete solution and must be coupled with other security measures to be effective. As with other security systems, poor maintenance can easily lead to a critical system failure.