Configuring networks cmd for ubuntu

 https://ubuntu.com/server/docs/configuring-networks

You can always use ubuntu UI to do it,

but to do it in cmd line way :

show gateway:

ip route show

default via 10.102.66.1 dev eth0 proto dhcp src 10.102.66.200 metric 100

10.102.66.0/24 dev eth0 proto kernel scope link src 10.102.66.200

10.102.66.1 dev eth0 proto dhcp scope link src 10.102.66.200 metric 100 

add default gateway:

sudo ip route add default via 10.102.66.1

add DNS
sudo vim /etc/resolv.conf

nameserver 8.8.8.8

nameserver 8.8.4.4