The second field gives the base URL of the source. Combined with the filenames listed in the <filename>Packages.xz</filename> files, it must give a full and valid URL. This can consist in a Debian mirror or in any other package archive set up by a third party. The URL can start with <literal>file://</literal> to indicate a local source installed in the system's file hierarchy, with <literal>http://</literal> or <literal>https://</literal> to indicate a source accessible from a web server, or with <literal>ftp://</literal> or <literal>ftps://</literal> for a source available on an FTP server. The URL can also start with <literal>cdrom:</literal> for CD-ROM/DVD/Blu-ray disc based installations, although this is less frequent, since network-based installation methods are eventually more common. More methods like <literal>ssh://</literal> or <literal>tor+http(s)://</literal> are supported and are either described in <citerefentry><refentrytitle>sources.list</refentrytitle><manvolnum>5</manvolnum></citerefentry> or their respective <emphasis role="pkg">apt-transport-<replaceable>method</replaceable></emphasis> package documentation.
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