in etc/network/interfaces:
iface eth0 inet static
address 192.168.1.248
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
allow-hotplug wlan0
auto wlan0
iface wlan0 inet static
address 192.168.1.248
netmask 255.255.255.0
network 192.168.1.0
gateway 192.168.1.1
dns-nameservers 192.168.1.1 # the router's IP
wpa-ssid "my network name"
wpa-psk "my password"