sudo vim /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 p5p1
iface p5p1 inet static
address 192.168.0.99
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1
——————————————————————————-

Since Ubuntu 12.04 the handling of resolv.conf changed.

If you want to add your own nameserver address(es) then edit the file /etc/resolvconf/resolv.conf.d/head
and add your nameserver there (eg: Google open dns)

sudo vim /etc/resolvconf/resolv.conf.d/head

——————————————————————————-
nameserver 8.8.8.8
nameserver 8.8.4.4
——————————————————————————-

save the file and then run

sudo resolvconf -u

restart network settings

sudo /etc/init.d/networking restart