ip route 0.0.0.0 0.0.0.0 172.16.3.1

This command sets a default route on a network device with a next hop IP address of 172.16.3.1. This means that any traffic that does not have a specific route in the routing table will be sent to this next hop IP address for further processing. The 0.0.0.0 0.0.0.0 destination network and netmask combination represents all possible destinations, or "everything".