This will be tricky to explain but I'll do my best! I recently got a certification and in the exam there was a part dealing with default routes. I ended up solving the section but I got lucky and I want to understand why my answer was correct.
We were on the 192.168.x.x network. We were supplied a PCAP that showed some traffic going to a website on the 10.10.x.x network. First thing I did was try to browse to the site - no luck. Next thing I did was check my routing table - no route to the 10.10.x.x network. Next I tried "ip route add 10.10.x.0/24 via 192.168.x.1" - nope! Still couldn't get to the site. At this point I didn't know what to do since I didn't know the IP for the default gateway for the 10.10.x.x network. I read through the PCAP some more and saw some ARP traffic between some devices on the 10.10.x.x network. I plugged those in to the routing table and eventually was able to access the site.
