sudo nano /etc/network/interfaces
auto eth0 iface eth0 inet static address 192.168.1.81 netmask 255.255.255.0 gateway 192.168.1.254
sudo rm /var/lib/dhcp/*
sudo reboot
ifconfig
References :
http://www.modmypi.com/blog/tutorial-how-to-give-your-raspberry-pi-a-static-ip-address