BBB red board network issues

Currently i am facing issues, i am using BBB red industrial board with Debian GNU/Linux 7.8 (wheezy) os and made to boot from SDcard and image date is BeagleBoard.org Debian Image 2015-03-01.

i set Static IP to bbb-Red board which is in /etc/network/interface file and its look like this…

cat /etc/network/interfaces

This file describes the network interfaces available on your system

and how to activate them. For more information, see interfaces(5).

The loopback network interface

auto lo
iface lo inet loopback

The primary network interface

#auto eth0
#iface eth0 inet dhcp

Example to keep MAC address between reboots

#hwaddress ether DE:AD:BE:EF:CA:FE
auto eth0
iface eth0 inet static
address 192.109.209.70
netmask 255.255.255.0
network 192.109.209.0
gateway 192.109.209.176
dns-nameservers 8.8.8.8
dns-nameservers 8.8.4.4

The secondary network interface

#auto eth1
#iface eth1 inet dhcp

WiFi Example

#auto wlan0
#iface wlan0 inet dhcp

wpa-ssid “essid”

wpa-psk “password”

Ethernet/RNDIS gadget (g_ether)

… or on host side, usbnet and random hwaddr

Note on some boards, usb0 is automaticly setup with an init script

#iface usb0 inet static
#address 192.168.7.2
#netmask 255.255.255.0
#network 192.168.7.0
#gateway 192.168.7.1

after rebooting bbb, its fine i can see and can do ssh to the BBB. After that i keep device on for the over night and when i came back at morning, i can’t do ssh / ping.
with using static ip of bbb then i did use debug port to find out its issues
then i found its ip address changed to other.
Can you tell why this is happing…??