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Suppose this hypothetical scenario: I have a parent network 192.168.0.0/16 and a subnetwork 192.168.42.0/24.

I have a router with two network interfaces in between, whose address in the subnetwork is 192.168.42.1. Can I assign this same address to the other network interface, the one connected to the parent network? Is it usual?

Daniel K
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lvella
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1 Answers1

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Yes, this is known situation. This is solved with proxy ARP.

Let's me depict where problem is. A smaller network, .42.0/24, won't have problems. But, systems in the .0.0/16 network consider addresses in .42.0/24 as part of their LAN and will try to access them directly, without any routers. They will send ARP requests, to which nobody can answer, because these requests are not routable.

Router in this case has to be smart, it will act as an ordinary router for its /24 LAN, but to the /16 network it should appear as a bunch of hosts. It will answer to each ARP request from /16 network for any IP address that belongs to /24 network (i.e. any .42.x address) with its own MAC address (of /16-side interface). That way all packets to the hosts with /24 addresses will be delivered to it and it could then route them to the proper destination.

Systems in the /24 LAN should have its /24-side address set as gateway. Which address is on the /16-side is not very important, because think of it, whatever it will be, the depicted scenario will work. So it is often set to the same address as /24-side, but with different network mask.

Nikita Kipriyanov
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