Did I misunderstand arp'ing and broadcasting

Warpig

Just got a question wrong on my Boson ExSim practice test (QID 501) that says that a Host will broadcast an arp request looking for a remote host's mac even if the destination IP address is not from the same subnet. I could swear I read in the Odom books that if the IP address is not in the same subnet there is no reason to arp and cause extra broadcast traffic on the subnet when the host isn't on that subnet and can't possibly answer due to it being in a different broadcast domain. This Boson Question's answer states that the host trying to resolve the remote hosts MAC will arp and if the Cisco router between the two subnets has an entry for the host in its arp table will answer the arp request in a process called "Proxy ARP" and the Router will return it's MAC address to the arp'ing host as a reliable path so the host will send the frame to the router to be delivered to the host on the other subnet. The way I understood from the Odom ICND1/2 3rd edition books is that the host will not bother to arp seeing the ip is in a different subnet and just encapsulate the packet and send it to the default gateway. If there is no arp from the host how can the router answer using a "Proxy Arp"? Now I'm really confused.

So what am I misunderstanding here?

Nick

Warpig

Bah... I know why. After reading on Cisco's site with an example of "Proxy ARP" I understand where I went wrong.
The issue is one "subnet" is 10.2.1.0/16 and the other is 10.2.2.0/16. I didnt pay enough attention to the subnet masks of the two "subnets". The host arps because it actually expects the other host to reply to the broadcast thinking that the other host is on the same "subnet" due to both "subnets" being a /16 mask and both being on the 10.2.0.0 network.

Nick

theodoxa

Sometimes, they will have a question where even though 2 hosts are on different physical subnets, they intentionally hosed up the subnetting so that the subnets overlap causing a device to believe that a device on the other subnet is actually on its own subnet and thus causing it to ARP.