BBB red board network issues

[Guru raj.M]

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...??