Sometimes its necessary that your machine gets a static IP-address. For example if you want to run a server or a other service on that machine. On Debian based machines you can do this by:

1. Edit /etc/network/interfaces (f.e.: with nano)
   

   auto lo
   iface lo inet loopback
   
   auto eth0
   iface eth0 inet static
           address 192.168.88.6
           netmask 255.255.255.0
           gateway 192.168.88.1
   
   auto wlan0
   allow-hotplug wlan0
   iface wlan0 inet static
           address 192.168.99.6
           netmask 255.255.255.0
           gateway 192.168.88.1
           wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
   
   iface default inet dhcp

   In this case we set the IP fix to 192.168.88.6 for both wlan and ethernet.

2.  Setup wireless lan settings (/etc/wpa_supplicant/wpa_supplicant.conf)

   ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
   update_config=1
   
   network={
           ssid="VERYNICENET"
           psk="PSKKEYINCLEARTEXT"
           proto=RSN
           key_mgmt=WPA-PSK
           pairwise=CCMP
           auth_alg=OPEN
   }

3. Disable network interface (wlan / eth)

   $> sudo ifdown wlan0
   $> sudo ifdown eth0

4. Enable it again to load changes

   $> sudo ifup wlan0
   $> sudo ifup eth0

5. You can skip step 3 and 4 if you just reboot your machine

   $> sudo reboot