The lookback address is a special, reserved IP address used by a device to send network traffic to itself.
Lookback ipv4 address: the entire 127.0.0.0/8 is reserved for lookback. It means any IP address from 227.0.0.0 to 127.255.255.255 routes back to the device. The most common used in the range is 127.0.0.1 which is also universely known as localhost.
Lookback ipv6 address: use a single, explicitly designed address for lookback: : :1(often written as 0:0:0:0:0:0:0:1)