 First introduced in FreeBSD version 4 , jails is a security mechanism and an implementation of operating @-@ system @-@ level virtualization that enables the user to run multiple instances of a guest operating system on top of a FreeBSD host . It is an enhanced version of the traditional chroot mechanism . A process that runs within such a jail is unable to access the resources outside of it . Every jail has its own hostname and IP address . It is possible to run multiple jails at the same time , but the kernel is shared among all of them . Hence only software supported by the FreeBSD kernel can be run within a jail . 
