Host without default gateway needs to reach host on external network

[jl81]

Hi,

A customer has a subnet, 192.168.0.0 /24. Hosts on that subnet has no default gateway configured, and by some reasons it's impossible to set the default gateway.

There is a router (Cisco 4331) connected to the subnet with ip 192.168.0.1.

The 4331 router is connected to another router, and behind that other router there is another subnet, 192.168.9.0/24.

We need the hosts on the 192.168.0.0/24 subnet (the hosts without default gateway) to be able to communicate with one server on the other network, with ip 192.168.9.20.

Is this possible using NAT? I mean, I would like to NAT an IP e.g 192.168.0.20 to 192.168.9.20. That way the hosts without gateway thinks they are communicating with 192.168.0.20, but they are actually speaking to the host on the other network.

I have tried creating static nat, and I get ping reply and arp entries for 192.168.0.20, but I guess it's the local router that responds to the pings.

[chrihussey]

I checked a 4331 and it looks like "proxy-arp" is enabled by default. That may be a possible solution and not require any additional configuration. As long as the 4331 has the route to the 192.168.9.20 host. You could also do a "debug arp" to see if it works.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_arp/configuration/15-mt/arp-15-mt-book/arp-config-arp.html#GUID-42FF9655-54E7-42D3-AA22-DF0B416E7CF6

[Harold Ritter]

You would need to change the subnet mask on the workstation for it to think that it is on the same subnet as the host on subnet 192.168.9.0/24. For example, 192.168.0.0/28 so that the workstation arp for station 192.168.9.20.

Regards,