Question About Proxy ARP
aderon
Member Posts: 404 ■■■■□□□□□□
in CCNA & CCENT
Sorry for all the questions lately, but I'm reading about Proxy ARP and this little tidbit has got me confused
From the book:
"If a Proxy ARP-enabled router receives an ARP request for an IP address that it knows isn't on the same subnet as the requesting host, it will respond with an ARP reply packet to the host"
When would a host ever send an ARP request for an IP address that isn't on its own subnet? From what I understand, the host would look at the destination IP. If the destination IP is on a different subnet, then it goes to the default gateway. If it doesn't know the default gateway's MAC address then it ARPs for that, but the default gateway's IP is always going to be on the same subnet as the host, which leads me back to my original question. Under what scenario would an ARP request be sent out for an IP address that isn't on the local subnet?
Edit: Think I just figured this out. They really need a delete thread button :P
From the book:
"If a Proxy ARP-enabled router receives an ARP request for an IP address that it knows isn't on the same subnet as the requesting host, it will respond with an ARP reply packet to the host"
When would a host ever send an ARP request for an IP address that isn't on its own subnet? From what I understand, the host would look at the destination IP. If the destination IP is on a different subnet, then it goes to the default gateway. If it doesn't know the default gateway's MAC address then it ARPs for that, but the default gateway's IP is always going to be on the same subnet as the host, which leads me back to my original question. Under what scenario would an ARP request be sent out for an IP address that isn't on the local subnet?
Edit: Think I just figured this out. They really need a delete thread button :P
2019 Certification/Degree Goals: AWS CSA Renewal (In Progress), M.S. Cybersecurity (In Progress), CCNA R&S Renewal (Not Started)
Comments
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NetworkVeteran
Member Posts: 2,338 ■■■■■■■■□□
Someone else will have the same question one day and find this thread.
Imagine you create a /22 network, e.g. 10.1.8.0 255.255.252.0, with hosts 10.1.8.1, 10.1.8.12, 10.1.9.20, etc. Two years later, you decide the network has too many hosts and you want to re-do it as /24s.. e.g., 10.1.8.0/24, 10.1.9.0/24, etc. Proxy ARP can be utilized during the transitional period while the router has a /24 mask but the hosts still have /22 masks until their DHCP leases expire or they're statically reconfigured. Proxy ARP also comes up in the context of NAT (Network Address Translation).
It also can allow bad configuration to "work" and some admins turn it off as general practice. -
t3mp
Member Posts: 8 ■□□□□□□□□□
ARP-req send in case:
HostA: fa0/0: 192.168.0.1/24, route: ip route 0.0.0.0 0.0.0.0 fa0/0, when HostA ping 1.1.1.1 it send ARP-req to 1.1.1.1 over fa0/0 to broadcast address, no MAC address of Default GW need.