This describes the ZyXEL modems that I have. On the (LAN-side-facing, modem to router cable) admin i/f the modem has an IPv4 address and netmask set up, but no default gateway address and IPv6 is not set up at all. I have not been able to find any way of declaring a default gateway address. I talk to the modem using fake source addresses chosen to be within the subnet thatbtye modem understands, as defined by its (addr, netmask) settings.
What exactly might happen when the modem tries to talk to an arbitrary other address, one that is not in the subnet? I think the possibilities are; 1. it might just give up and report an error internally and so do nothing, because it knows it has no default gateway. 2. it ARPs for something and that something would be the ip address of a default value for the default gateway 3. it ARPs for the destination address, treating it as if it were on-link, regardless of the fact that it is not inside the defined netmask. The latter does not sound very likely. If I were the designer concerned, 1 is not useful, 2 is not useful at all because the default default gateway value is bound to be wrong, so that just leaves 3, bizarrely, so maybe that is the only useful thing you could do.
I wonder if I could find out what it is doing? If 1 then I am out of luck, but with 2 I am in luck because my router could impersonate that default default gateway on the relevant interface, and with 3 I would just need to get my router to forward everything from the modem to my main LAN.
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Topic: Modem-router in modem-only mode, with no default gateway setup for admin i/f (Read 739 times)