CompTIA Network+ Practice Test

What happens to packets when a route to the destination network does not exist?

1. The packet is queued for later delivery

2. The packet is forwarded to a default route

3. The packet is dropped and an ICMP host unreachable message is sent

4. The packet is sent back to the sender

The correct answer is: The packet is dropped and an ICMP host unreachable message is sent

When there is no existing route to the destination network, the most appropriate action taken by routers within the network is to drop the packet and generate an Internet Control Message Protocol (ICMP) host unreachable message. This process occurs because routers maintain a routing table that lists all known routes. If the destination network is not found in this table, the router cannot determine where to forward the packet. Dropping the packet helps maintain network efficiency by preventing congestion that could arise from continuously attempting to forward packets to nonexistent addresses. The ICMP host unreachable message serves two important purposes: it informs the sender that the intended destination cannot be reached, and it provides diagnostic insights into the connectivity of the network. This feedback is crucial, especially for applications that may need to handle errors or retry sending packets. In contrast, the other options do not accurately describe the standard behavior of routers when they encounter a packet destined for an unreachable network. For example, queuing a packet for later delivery would imply that there might be a chance of the route becoming available, which is not the case here. Forwarding to a default route might be suggested when a partial match in routing is found, which is not applicable when there is no route at all. Lastly, sending the packet back to