A dedicated Debian packages proxy will ensure you that the package you required to install your Linux distribution one day will always be available to other installations.
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Using a non dedicated proxy like Squid with a very large cache, all the packages downloaded from the Internet will be stored temporally into the hard disk and then made available to all the computers running behind your proxy. With this solution, only the .deb files you really need will be downloaded no bandwidth and no space is wasted! Only the first installation will be “Internet slow”, the next one will be done at LAN speed. But if you have to install your distribution some time later, all the package you had stored into the Squid cache will probably have been replaced by other objects while you were surfing on the Net: you will need to download every package again.