debootstrap is a tool which will install a Debian base system into a subdirectory of another, already installed system. It doesn’t require an installation CD, just access to a Debian repo.
It can also be installed and run from another operating system, so, for instance, you can use debootstrap to install Debian onto an unused partition from a running Gentoo system.
It can also be used to create a rootfs for a machine of a different architecture, which is known as “cross-debootstrapping”.
To setup a stable system:
mkdir /stable-chroot
debootstrap stable /stable-chroot http://deb.debian.org/debian/To setup an Ubuntu system from Debian:
mkdir /ubuntu_xenial_1604
debootstrap --arch=amd64 xenial /ubuntu_xenial_1604 http://archive.ubuntu.com/ubuntu/