Many host systems connected to the Internet run a version of the UNIX operating system. TCP/IP was first implemented in the early 1980's for the version of UNIX written at the University of California at Berkeley known as the Berkeley Software Distribution (BSD). Many modern versions of UNIX derive their networking code directly from the BSD releases, thus UNIX provides a more-or-less standard set of TCP/IP services. This standard of sorts has resulted in many different versions of UNIX suffering from the same vulnerabilities, however it has also provided a common means for implementing firewall strategies such as IP packet filtering. It is important to note that BSD UNIX source code is fairly easy to obtain free from Internet sites, thus many good and bad people have been able to study the code for potential flaws and exploitable vulnerabilities.
Although UNIX is the predominant Internet host operating system, many other types of operating systems and computers are connected to the Internet, including systems running Digital Equipment Corporation's VMS, NeXT, mainframe operating systems, and personal computer operating systems such as for DOS, Microsoft Windows, and for Apple systems. Although personal computer systems often provide only client services, i.e., one can use TELNET to connect from but not to a personal computer, increasingly powerful personal computers are also beginning to provide, at low cost, the same services as larger hosts. Versions of UNIX for the personal computer, including Linux, FreeBSD, and BSDi, and other operating systems such as Microsoft Windows NT, can provide the same services and applications that were, until recently, found only on larger systems. The ramifications of this are that more people are able to utilize a wider array of TCP/IP services than ever before. While this is good in that the benefits of networking are more available, it has negative consequences in that there is more potential for harm from intruders (as well as uneducated but well-intentioned users who, to some sites, may appear to be intruders).