Strange enough – there are a lot of CLOSED SOURCE firmware out there – be it realtek or be it intel Wifi-Adapters.

Per default Debian will NOT install non-free firmware which is because of privacy reasons.

Vendors kind of „not care“ about OpenSource community trying to avoid such firmwares?

steps to get going, unless you do not want Wifi…

0. identify your system -> here are all Version-Numbers and „Codenames“ for that Version.

su; # become root

cat /etc/debian_version; # identify your version of debian
8.6 = Jessie

uname -a
Linux 3.16.0-4-686-pae #1 SMP Debian 3.16.36-1+deb8u1 (2016-09-03) i686 GNU/Linux

1. change sources.list to include non-free firmware

vim /etc/apt/source.list; # edit sources list

# at the end of the file put:

deb jessie main contrib non-free
deb-src jessie main contrib non-free

2. identify your network / wifi adapter:

apt-get install lshw; # install

lspci -knn|grep -EiA2 net;

00:19.0 Ethernet controller [0200]: Intel Corporation 82577LM Gigabit Network Connection [8086:10ea] (rev 06)
    Subsystem: Lenovo Device [17aa:2153]
    Kernel driver in use: e1000e
03:00.0 Network controller [0280]: Intel Corporation Centrino Ultimate-N 6300 [8086:4238] (rev 35) <- this is it
    Subsystem: Intel Corporation Centrino Ultimate-N 6300 3x3 AGN [8086:1111]

0d:00.0 SD Host controller [0805]: Ricoh Co Ltd MMC/SD Host Controller [1180:e822] (rev 01) <- this is not a network card :-D

lshw -class network; # show all network cards
       description: Ethernet interface
       product: 82577LM Gigabit Network Connection
       vendor: Intel Corporation
       physical id: 19
       bus info: pci@0000:00:19.0
       logical name: eth0
       version: 06
       serial: f0:de:f1:33:82:ec
       size: 100Mbit/s
       capacity: 1Gbit/s
       width: 32 bits
       clock: 33MHz
       capabilities: pm msi bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=e1000e driverversion=2.3.2-k duplex=full firmware=0.12-1 ip= latency=0 link=yes multicast=yes port=twisted pair speed=100Mbit/s
       resources: irq:45 memory:f2600000-f261ffff memory:f2625000-f2625fff ioport:1820(size=32)
  *-network UNCLAIMED
       description: Network controller
       product: Centrino Ultimate-N 6300
       vendor: Intel Corporation
       physical id: 0
       bus info: pci@0000:03:00.0
       version: 35
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress cap_list
       configuration: latency=0
       resources: memory:f2000000-f2001fff

3. search for what firmware package would need to be installed:

-> this kind of tells me, one needs to search for a package called „iwlwifi“

4. install the missing firmwrae

apt-cache search iwlwifi; # search for package

firmware-iwlwifi - Binary firmware for Intel Wireless cards
apt-get update && apt-get install firmware-iwlwifi; # install that package

if your adapter is realtek:

apt-cache search realtek; # search for package

flashrom - Identify, read, write, erase, and verify BIOS/ROM/flash chips
nictools-pci - Diagnostic tools for many PCI Ethernet cards
librtlsdr-dev - Software defined radio receiver for Realtek RTL2832U (development files)
librtlsdr0 - Software defined radio receiver for Realtek RTL2832U (library)
rtl-sdr - Software defined radio receiver for Realtek RTL2832U (tools)
firmware-realtek - Binary firmware for Realtek wired and wireless network adapters

apt-get update && apt-get install firmware-realtek; # install realtek firmware

5. install a network manager application (there is none installed per default under gnome2/MATE)

apt-get install network-manager-gnome; # for conveniently managing wifi connections from your MATE / Gnome2 Desktop

# when done do

reboot; # reboot system hardcore

you should have a nice icon in the upper right corner: