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I have an appliance with a class C IP address, for example I can't or don't want to change this IP address. However when I connect this appliance to my network I want it to appear to have a different IP address for example

Is there any network device which I can plug this into which will change the IP address in the packets on the fly, so that the appliance can appear to have a different IP address?

This would be sort of analogous to what a NAT router does with packets.

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You answer your own question what you need is a device doing NAT.
Generally, most firewall and router provide NAT feature.

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+1 hit the nail on the head. – sybreon Sep 10 '09 at 9:35
Not really what I was looking for, but perhaps I wasn't totally clear! The appliance runs various services such as VNC and windows remote desktop, and expects to be able to grab files via FTP. If I only had a single appliance, disabled DHCP in the NAT router, and configured the NAT router tp have the appliance in the DMZ, then this would work. But what happens when I need more than one appliance? I suppose I could always have more than one NAT router, but I was hoping to be able to have a single device with some kind of IP address mapping table. – Anonymous Oct 15 '09 at 13:52
I'm not following you, you can NAT more than one IP on a router. So you can have has many appliance has you want. – radius Oct 15 '09 at 15:03
It is what you are looking for. You are going to need to get a router/firewall device that is not designed for home users. Or just get one of those, and re-flash it openwrt. Either way you need a real firewall. – Justin Feb 20 '10 at 18:48


You can think of the IP address as the street address of a hotel. The different services use different TCP or UDP ports, like different hotel rooms.

If your router can do plain "static" NAT, it can give the entire building a new address. This doesn't change the room numbers, so all services are still available. That may be what you want.

The reason it gets confusing is that many routers, especially for home and small business, make it hard or impossible to change the (IP) address of the entire hotel. They prefer to move several rooms to another hotel down the street, with a different address. To improve sysadmin job security (and perhaps network security), the port numbers on the rooms may also change when they move (Port Address Translation - PAT).

That's why you may need to create access rules individually for each of the different services--rooms or groups of rooms--on the appliance. And you may need different settings for outbound connections (the appliance opens an FTP session to grab a file), than for inbound connections (you control the appliance from the Internet via VNC).

To dive a bit deeper, try the section about NAT firewalls here.

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Weirdest. analogy. ever. – Justin Feb 20 '10 at 18:43
Yep, weird is what went through my mind as well. Correct perhaps, but hard to read. "The reason it gets confusing" is because of analogies. – John Gardeniers Feb 20 '10 at 20:48

I'm not sure if you're specifically looking for an external NAT style device or if that was just a suggestion/thought. It would've also helped if you could reveal more information regarding the purpose of this configuration, what are you trying to achieve with the different address? masquerade? security? fast deployment?

How about using a second address on the same interface?


Right click the network interface >> properties >> Internet protocol version 4 >> properties >> Alternate configuration


ip addr add X.X.X.X/YY dev eth0

See your distribution's documentation for a thorough explanation on configuring multiple addresses on the same interface.

Don't forget to add routes with appropriate metrics as well.

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