Almost any network interface card (NIC) supported by the Linux kernel should also be supported by the installation system; modular drivers should normally be loaded automatically.
The installation system currently does not support retrieving firmware. This means that any network cards that use a driver that requires firmware to be loaded, is not supported by default.
If there is no other NIC you can use during the installation, it is still possible to install Debian GNU/Linux using a full CD-ROM or DVD image. Select the option to not configure a network and install using only the packages available from the CD/DVD. You can then install the driver and firmware you need after the installation is completed (after the reboot) and configure your network manually. Note that the firmware may be packaged separately from the driver and may not be available in the “main” section of the Debian GNU/Linux archive.
If the driver itself is supported, you may also be
able to use the NIC during installation by copying the firmware from some
/usr/lib/hotplug/firmware. Don't forget to
also copy the firmware to that location for the installed system before
the reboot at the end of the installation.
Wireless NICs are in general supported as well, with one big proviso. A lot of wireless adapters require drivers that are either non-free or have not been accepted into the official Linux kernel. These NICs can generally be made to work under Debian GNU/Linux, but are not supported during the installation.
If there is no other NIC you can use during the installation, it is still possible to install Debian GNU/Linux using a full CD-ROM or DVD image. Use the same procedure as described above for NICs that require firmware.
In some cases the driver you need may not be available as a Debian package. You will then have to look if there is source code available in the internet and compile the driver yourself. How to do this is outside the scope of this manual.