Configuration of the Advanced Packaging Tool (APT) system repositories is stored in the /etc/apt/sources.list file and the /etc/apt/sources.list.d directory. An example of this file is referenced here, along with information on adding or removing repository references from the file.

You may edit the file to enable repositories or disable them. For example, to disable the requirement of inserting the Ubuntu CD-ROM whenever package operations occur, simply comment out the appropriate line for the CD-ROM, which appears at the top of the file:

# no more prompting for CD-ROM please
# deb cdrom:[Ubuntu 16.04 _Xenial Xerus_ - Release i386 (20111013.1)]/ xenial main restricted

Extra Repositories

In addition to the officially supported package repositories available for Ubuntu, there exist additional community-maintained repositories which add thousands more packages for potential installation. Two of the most popular are the Universe and Multiverse repositories. These repositories are not officially supported by Ubuntu, but because they are maintained by the community they generally provide packages which are safe for use with your Ubuntu computer.

Packages in the Multiverse repository often have licensing issues that prevent them from being distributed with a free operating system, and they may be illegal in your locality.

Be advised that neither the Universe or Multiverse repositories contain officially supported packages. In particular, there may not be security updates for these packages.

Many other package sources are available, sometimes even offering only one package, as in the case of package sources provided by the developer of a single application. You should always be very careful and cautious when using non-standard package sources, however. Research the source and packages carefully before performing any installation, as some package sources and their packages could render your system unstable or non-functional in some respects.

By default, the Universe and Multiverse repositories are enabled but if you would like to disable them edit /etc/apt/sources.list and comment the following lines:

deb xenial universe multiverse
deb-src xenial universe multiverse

deb xenial universe
deb-src xenial universe
deb xenial-updates universe
deb-src xenial-updates universe

deb xenial multiverse
deb-src xenial multiverse
deb xenial-updates multiverse
deb-src xenial-updates multiverse

deb xenial-security universe
deb-src xenial-security universe
deb xenial-security multiverse
deb-src xenial-security multiverse