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I know this shouldn't work but using Cisco's Packet Tracer, I'm able to send a ping from PC2 to PC1.

in reality, PC2 is on a different network and thinks that PC1 is on the same network, thus, sending an ARP requesting the mac of 10.0.9.7 using a broadcast. this should fail. am I right?

Can PC2 from 10.0.8.2 send a ping to 10.0.9.7? it's working on packet tracer and I don't know why.

  • PC2 shouldn't be able to send traffic to PC1 (and vice versa) for two reasons. 1. PC1 and PC2 are physically separated by a router. PC2, however, would think that PC1 is local due to the mask and wouldn't try to route the packets. 2. No matter the subnet mask, PC1 and PC2 on different broadcast domains due to the router. PC2 wouldn't be able to get PC1s MAC address. – pooter03 Jun 26 '15 at 14:50
  • "No matter the subnet mask", if PC2 has a subnet mask of /24, any packet sent from PC2 to a host on a different subnet will go through the Router, thus it will use the router's MAC as the destination MAC and the router will forward the packet to the right network (it will send an arp to get PC1's MAC and send it the designated packet). – TCP_Explorer Jun 26 '15 at 15:13
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If your router is running Proxy ARP (and I think Cisco does by default), it will answer ARP requests, even for IPs that don't properly belong to the router, and then forward the IP packet based on its routing table. It's probably answering PC2's ARP query, and once PC2 sends the ICMP packet to Router0 (thinking that Router0 has the MAC address for 10.0.9.7), then Router0 forwards the packet appropriately.

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