The virtual hosts on Telehack all run operating systems. The operating system can impact certain things for the host, such as rootkit support programs, compatibility for different programs, and account creation exploit programs. The host operating systems were all actual operating systems during the 1980's and 1990's, which is the time period which Telehack simulates. The major host operating syatems are the following: ATT, AIX, AUX, BSD, Dynix, HP-UX, IBM, MACH, MIL, SunOS, SysV, Ultrix, VMS, and Xenix. There are also some special operating systems which only a single host runs.