##/etc/apt/sources.list
deb http://www.backports.org/debian/ sarge-backports main

##/etc/apt/preferences
Explanation: For backports.org packages
Package: *
Pin: origin backports.org
Pin-Priority: 100

Explanation: For Nagios2.5 Backport
Package: nagios2
Pin: origin backports.org
Pin-Priority: 999

Explanation: For Nagios2.5 Backport
Package: nagios2-common
Pin: origin backports.org
Pin-Priority: 999

##an apt-cache policy command...
apt-cache policy nagios2
nagios2:
  Installed: (none)
  Candidate: (none)
  Package Pin: (not found)
  Version Table:
     2.5-1~bpo.1 999
          1 http://www.backports.org sarge-backports/main Packages

##an apt-get install...
apt-get install nagios2
Reading Package Lists... Done
Building Dependency Tree... Done
Package nagios2 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package nagios2 has no installation candidate