See https://www.debian.org/ for information about Debian GNU/Linux.

This FTP site is a repository for old debian releases. For new releases
please see https://deb.debian.org/debian/.

Releases are stored by their codenames under the dists/ directory.
  bullseye is Debian 11
  buster   is Debian 10
  stretch  is Debian 9
  jessie   is Debian 8.0
  wheezy   is Debian 7.0
  squeeze  is Debian 6.0
  lenny    is Debian 5.0
  etch     is Debian 4.0
  sarge    is Debian 3.1
  woody    is Debian 3.0
  potato   is Debian 2.2
  slink    is Debian 2.1
  hamm     is Debian 2.0
  bo       is Debian 1.3
  rex      is Debian 1.2
  buzz     is Debian 1.1
  
If you are using APT the relevent sources.list entries are like:
  deb https://archive.debian.org/debian/ $RELEASE main contrib
for example:
  deb https://archive.debian.org/debian/ sarge main contrib