- The Proxy ARP occurs, if one node will respond to an ARP request on behalf of another node.
- When a host or node is communicating with another host on the same network, then the host will send ARP request to the other host’s IP address.
- If a host or node is communicating with another host on a different network, then the host will send ARP request to the Default Gateway’s IP address.
- You know that already, Subnet Mask will tell, another IP address is on the same network or a different network.
- Check the below topology:
- PC1 has IP address 20.0.0.11 with subnet mask of 255.255.255.0. So any IP address from the range of 20.0.0.0 – 20.0.0.255 will be consider as local network for the PC1.
- PC2 has IP address 20.0.0.22 and mis-configured with a subnet mask of 255.255.0.0. So any IP address from the range of 20.0.0.0 – 20.0.255.255 will be consider as local network for PC2
- Suppose both of these PC's are trying to communicate with PC3, which is on a different network and has the IP address 20.0.4.44.
- When PC1 tries to communicate with 20.0.4.44, it will consider PC3 on a different network and will send the ARP packet to the default gateway.
- And when PC2 tries to communicate with 20.0.4.44, it will consider PC3 on the same network and will send ARP packet to PC3’s MAC address directly.
- PC2’s ARP Request will be broadcast to the local network, but will never send to Router1 to PC3, and it will not get any response, so PC2 will not able to communicate with PC3.
- Unless the Router1 will respond to PC2’s ARP Request on behalf of PC3 – Thi is the Proxy ARP.
- So the misconfigured Subnet Mask, PC2 can communicate with PC3, via Router’s Proxy ARP.
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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