Time to describe the solution to my last blog post.

Let’s see the topology again:

The router does not have any routing protocol configured. It does have all interfaces configured: 10.0.0.1 towards the WAN cloud, 10.0.1.1 towards the PC 2 subnet, and an IP address towards the PC 1 subnet. PC 2 is a Windows computer, static IP 10.0.1.2. PC 1 is a Windows computer set to DHCP client, but there is no DHCP pool configured on the router or any other device. PC 1 hasn’t had any special changes to the software settings or TCP/IP stack.

Under what circumstances will PC 1 and PC 2 be able to ping each other?

Well, note that I speak of Windows computers. This OS does a very specific thing when there’s no DHCP server: it will use an APIPA address. This is an address in the range 169.254.0.0/16. The remaining 16 hostbits are generated from the MAC address. So despite that I not explicitly give the PC1 IP address, you know in which subnet the IP will be. So the router interface towards PC1 needs an IP in the same subnet. Congrats to Pape Diack for working that out!

That’s half of the challenge. But to ping PC1 from another subnet, it’ll need a default route, which it does not have. Unless it somehow assumes the echo request originated from the local subnet.

First solution: a (static) NAT rule on the router, which translates an IP on another subnet to an IP on the local subnet. For PC1, it seems as if the router sends the echo request. Once the router receives the echo reply, it is translated and sent to PC2.

Second solution: proxy-arp. Enabled by default on a Cisco router. If PC1 doesn’t have a gateway, it desperately tries to ARP for PC2’s MAC address. If the router knows how to reach PC2 (possible through the connected subnet), it will reply with his own MAC, and forward the packet once it arrives.

A third, more out-of-the-box thinking solution was using IPv6, as suggested by some. Granted it would work, but not exactly what I was looking for. I better add more detail to my questions the next time.

Thanks for all the answers!

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