$ ip netns exec ns1 ip link set lo up
 $ ip netns exec ns1 ip link set eth1 up
 $ ip netns exec ns1 ip addr add 192.168.1.123/24 dev eth1
 $ ip netns exec ns1 ip -f inet addr show
 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
 3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    inet 192.168.1.123/24 scope global eth1
       valid_lft forever preferred_lft forever

 $ ip netns exec ns1 ip route add default via 192.168.1.1 dev eth1
 $ ip netns exec ns1 ping -c2 8.8.8.8
 PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
 64 bytes from 8.8.8.8: icmp_seq=1 ttl=51 time=22.1 ms
 64 bytes from 8.8.8.8: icmp_seq=2 ttl=51 time=20.1 ms