One of the tools used to install packages on a &debian; system is a program called apt-get, from the apt package Note that the program which actually installs the packages is called dpkg. However, this program is more of a low-level tool. apt-get is a higher-level tool, which will invoke dpkg as appropriate. It knows how to retrieve packages from your CD, the network, or wherever. It is also able to automatically install other packages which are required to make the package you're trying to install work correctly. . Other front-ends for package management, like aptitude and synaptic, are also in use. These front-ends are recommended for new users, since they integrate some additional features (package searching and status checks) in a nice user interface. In fact, aptitude is now the recommended utility for package management. apt must be configured so that it knows from where to retrieve packages. The results of this configuration are written to the file /etc/apt/sources.list. You can examine and edit this file to your liking after the installation is complete. If you are installing at default priority, the installer will largely take care of the configuration automatically, based on the installation method you are using and possibly using choices made earlier in the installation. In most cases the installer will automatically add a security mirror and, if you are installing the stable distribution, a mirror for the volatile update service. If you are installing at a lower priority (e.g. in expert mode), you will be able to make more decisions yourself. You can choose whether or not to use the security and/or volatile update services, and you can choose to add packages from the contrib and non-free sections of the archive. One question that will be asked in most cases is whether or not to use a network mirror as a source for packages. In most cases the default answer should be fine, but there are some exceptions. If you are not installing from a full CD or DVD or using a full CD/DVD image, you really should use a network mirror as otherwise you will end up with only a very minimal system. However, if you have a limited Internet connection it is best not to select the desktop task in the next step of the installation. If you are installing from a full CD or using a CD image (not DVD), using a network mirror is not required, but is still strongly recommended because a single CD contains only a fairly limited number of packages. The installer currently does not support using multiple CD or DVD images during the installation Adding that option is planned. . If you have a limited Internet connection it may still be best to not select a network mirror here, but to finish the installation using only what's available on the CD and install additional packages after the installation (i.e. after you have rebooted into the new system). If you are installing from a DVD or using a DVD image, the need to use a network mirror is a lot smaller, but there is still a chance that some packages that are part of the tasks you select in the next step will not be included on the first DVD. This is especially true if you are installing in another language than English: a number of font and localization packages are known to be on the second DVD. So, if you have a reasonable Internet connection it is still advisable to use a network mirror. To install additional packages after the installation you have two options: if you have additional CD/DVD images available (from same set as the installation CD/DVD you are using), you can add those to the sources.list using apt-cdrom; manually add a mirror to the sources.list using an editor. You can then use one of the package management front-ends to select and install additional packages It is possible to add both additional CDs or DVDs and a network mirror in /etc/apt/sources.list. Also adding a network mirror has the advantage that it will make updates of packages in point releases of the distribution available for installation. . In summary: selecting a network mirror is generally a good idea, except if you do not have a good Internet connection. If the current version of a package is available on the CD/DVD, the installer will alway use that. The amount of data that will be downloaded if you do select a mirror thus depends on a) the tasks you select in the next step of the installation, b) which packages are needed for those tasks, and c) which of those packages are present on the CD/DVD. Note that even if you choose not to use a network mirror, some packages may still be downloaded from the Internet if there is a security or volatile update available for them and those services have been configured.