Editar o arquivo /etc/network/interfaces:

  auto eth0
  iface eth0 inet static
      address 192.0.2.7
      netmask 255.255.255.0
      gateway 192.0.2.254
      

Executar :

# ifdown eth0
# ifup eth0

OBS: sustituir eth0 pela interface (checar com os comandos ifconfig ou ip addr.