The Internet as the merger of two major technologies computers and communication is a network of networks of computers, which plays a vital role in the Information and Communication Technology (ICT). It’s an emerging need of today’s high tech society as almost all requirements like education, entertainment, business, social services etc. of human beings are efficiently implementable through Internet. In present book, some Internet applications such as multicast, reliable transmission of unicast and multicast IP packets using UDP, generation of secret group-key, secured transmission of E-mails, secure multicast and enhancement of SET (Secure Electronic Transaction) have been investigated so that secured and reliable transmission through unreliable Internet is possible, and which are the vital need of the present-day scenario. Although all these enhancements have been done with respect to IPv4 ,they are equally applicable to IPv6, and it is true that IPv6 cannot be implemented overnight and instead, IPv4 will continue, although there are different opinions, next 20 to 25 years, an investigation of the same has also been done in book.