Combine 1-to-1 NAT with Dynamic NAT

Hello everyone,

I have a tricky situation and I am not quiet sure how to accomplish that.
There is one interface (eth1) on our Watchguard which is connected to an external router with a special internet connection.
Every connection to this interface (eth1) needs to be translated to an IP address in the subnet because only addresses from this subnet can be routet there. Currently I have a 1-to-1 NAT rule which translates our LAN subnet to
But now we need to extend our to a /23 subnet. So I would like to do a dynamic NAT for all connections to the interface eth1. Every connection could do a dynamic NAT to a single IP address in the subnet, e.g. Is this possible?



  • Options

    On the Dynamic NAT setup, you can specify "Set source IP addr" which should resolve your issue.

    Dynamic NAT entry:
    From: your source subnet, source Interface name, etc.
    To: eth1 Interface name
    Set source IP addr:

    Move this entry to the top of the list.
    Remove the 1-to-1 NAT entry no longer needed

  • Options

    If have already tried this but the web ui won't let me. It shows the following message:"The source IP address you specifiy must be on the same subnet as the primary or secondary IP address of the outgoing interface or the loopback interface". Should I add the as secondary IP address to the interface eth1 as workaround or will things break?

  • Options

    We need more details about the need for a IP addr here.
    Do you have a subnet anyplace in your config including
    If not, then adding as a secondary on eth1 should work - as long as whatever is down eth1 knows to route things back to eth1 for dest packets of, as presumably eth1 has a different subnet IP addr.

  • Options

    Yes, forgot to mention that eth1 has a different subnet IP address. I have added the as a secondary IP address to interface eth1 and now it works. Thank you.

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