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Given multiple systems (embedded devices) on a LAN with a factory-set default IP of 192.168.1.1/255.255.255.0, is it possible to communicate with them using UDP broadcast and to receive an answer for each of them (identified by a guid generated by the system)?

Our goal is to change the IPs on each system from a central location/computer on the same network by broadcasting a command with the new IP paired with the guid.

It it doable? My concerns are that the duplicate IPs may cause problems for switches etc.

Thanks in advance,

Per

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2 Answers

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If they're hard-coded with a single IP address then other than per-port VLANing and routing/NATing you're going to run into a lot of problems. They won't cause problems from a switching perspective as IP is L3, but they will cause problems for routing.

Can you not just set them to use DHCP? The MAC is already a GUID of sorts. If needs be you could then script a more permanent IP change from that point.

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DHCP is of course desirable, but mostly not available on the networks our systems operate (airport/airside), thus the need for an alternative way of setting the IPs. We could of course set the IPs by visiting each system physically, but automating this would give huge benefits during commissioning. So if I understand you correctly, it isn't feasible? I've seen similar solution being used for some serial-to-ethernet converters (Tibbo, Moxa ), but perhaps they are not using IP? – Per Aug 23 '11 at 9:24
Certainly you could use UDP to do this but I'm unaware of anything off the shelf to do this. – Chopper3 Aug 23 '11 at 9:38
I'll do the development myself, I'm just not a guru on networking so I needed to discuss the idea a bit before venturing further. Thanks! – Per Aug 23 '11 at 9:50
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You say "broadcasting a command" to change the ip address. Is this something the devices already implement or something new? If all these systems are in 1 subnet it is "possible" but would require a lot of layer 2 programming skills. Remember, a host isn't going to keep a list of MAC addresses it has seen for "no reason". Essentially, you would need to write a ton of code and have it all operate at a very low level in the host.

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We're about to implement the functionality in the devices - provided it is feasible. The idea is to send a UDP broadcast, assuming the end devices see it since they are on the same subnet. – Per Aug 23 '11 at 10:15
If you send a broadcast how is each device going to know which this broadcast is aimed at? – Aaron Aug 23 '11 at 10:34
First I'll send a "discovery command" that all devices will answer, supplying their UID. Then I can send a new IP paired with the UID; all systems will see it, but only one will match. – Per Aug 23 '11 at 10:48
The way to approach this sort of thing is to actually think "I'm just communicating with random MAC addresses". Later on you send them a special packet, again, dealing with a MAC address, and they "do something". If you're only connecting with them via MAC layer and not IP layer duplicate IPs won't enter into it because nothing will use those IPs to communicate. Again, if they're on a subnet 3 hops away this won't work because you've got to have a layer 3 packet to pass through a router. – Aaron Aug 23 '11 at 12:07
Our requirement is a single sub net only. I've done some testing now and it does seem to work the way I want it. – Per Aug 25 '11 at 9:28
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