Beginner Question: I just read an article talking about how the internet works and how routers make sure that packets arrive at the destination. However there is one thing that is not explained anywhere I am looking: What happens if we try to send a packet to a destination that does not exist?
Will it reach the NSP backbone and just be dropped?
Will the sender be informed about it or will we just wait for a timeout?
Follow up in case we just wait for a timeout: How do we know if the service does not exist or if we just timed out because it is busy?
Quote from the article:
"When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will begin its journey 'downward' through smaller and smaller networks until it finds it's destination."
Best Answer
There are several possibilities, depending if it is just a host not assigned to the address, or if the network does not exist on the Internet.
If your business has the full Internet routing table in it router and the route does not exist, the router will drop the packet and send an ICMP message back to the source host informing it that there is no path to the network.
If your business uses a default route to its ISP, then the default route matches every destination network, and the packet will be sent to the ISP router to which the default route points. The ISP router probably does not have a default route, and if the network does not exist in its routing table, the router will drop the packet and send an ICMP message back to the source host informing it that there is no path to the network.
This continues all along the path toward the destination network.
If the destination network exists and is advertised on the Internet, but there is no host assigned to the address, then the final router will send an ICMP message back to the source host informing it that the destination does not exist.