ifconfig - configure a network interface
[SYNOPSIS] ifconfig INTERFACE ADDRESS
  show status of active interfaces
# ifconfig
  enable an interfce
# ifconfig eth0 up
  disable an interface
# ifconfig eth0 down
  change IP address of an interface
# ifconfig eth0 192.168.0.10
  change subnet-mask of an interface
# ifconfig eth0 netmask 255.255.255.248