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The installer does require Internet access for installing pre-requisites using apt-get, yum and CPAN (but we do have a few suggestions for dealing with a system where Internet access is restricted).
As of 8.5.6G using the installer is the only supported method for installing or upgrading NMIS, because by now it is now suitably mature and robust, and because it's very very easy to miss crucial operations in a totally manual installation. Please see the Advanced Installer Use section at the bottom if you need more precise control over the installer.
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