Source : http://archive.debian.org/debian-archive/debian/README

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

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

Releases are stored by their codenames under the dists/ directory.
  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 http://archive.debian.org/debian/ $RELEASE main contrib
for example:
  deb http://archive.debian.org/debian/ sarge main contrib

/etc/apt/sources.list

deb http://archive.debian.org/debian-archive/debian/ lenny main contrib non-free
deb http://archive.debian.org/debian-security/ lenny/updates main contrib non-free