Proxy ARP

What this Proxy ARP networking question tests

This is a hard networking question that probes deep understanding of the Address Resolution Protocol and how it behaves in multi-subnet environments. It is the kind of question asked in interviews for network engineering, systems infrastructure, and low-latency trading infrastructure roles where candidates need to reason about layer 2 and layer 3 interaction.

Proxy ARP scenarios test whether you understand when and why an intermediate device (typically a router or gateway) responds to an ARP request on behalf of another host, rather than allowing the request to go unanswered or be forwarded. This requires clear mental models of ARP scope, subnet boundaries, and the distinction between the device making the request and the device that can legitimately answer it. Candidates who work through these problems typically sketch out the network topology, identify which hosts and devices can see which ARP frames, and reason about the consequences of a proxy responder's behaviour.

• ARP request/reply mechanics and broadcast scope
• Subnet masks and IP routing decisions
• Router and gateway behaviour across subnets
• When and why proxy ARP is enabled or disabled
• Gratuitous ARP and ARP cache consistency