1.1.d Explain IP operations

1.1.d [i] ICMP unreachable, redirect

If a router or a layer 3 switch receives a non-broadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the software receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it will send an ICMP host unreachable message to the source. This feature is enabled by default.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 996-999).  . Kindle Edition.