arp across subnets which devices can be detected?

Question

i have some devices connected in the same physical network (LAN) but in different subnets. I want to detect them all with the ARP from my computer. My Computer is also in the same physical network but on a different subnet.
! The devices have no arp protocol installed (yet).

Is it possible to detect the devices (with or withour ARP installed on every device)?
If the devices have a MAC-address but no IP-address, can ARP still detect the machines?

[I'm not sure that you fully understand what ARP is](http://en.wikipedia.org/wiki/Address_Resolution_Protocol). You don't *install* the ARP protocol. Any device that has an IP stack on it will by definition implement the ARP protocol. Oh, and by the way, ARP is a broadcast protocol, meaning it won't cross subnet boundaries. An ARP request for a device on another subnet will give you the MAC address of the gateway to that network. — Chris McKeown, Oct 12 '12 at 07:58

score 1 · Accepted Answer · answered Oct 12 '12 at 08:05

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Yes. If the devices have an assigned IP address, they will respond to ARP queries. (It may take some effort to properly craft the ARP queries.)
No. If the devices don't have an assigned IP address, they will not respond to ARP queries.

answered Oct 12 '12 at 08:05

David Schwartz

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Hi and thx. what do you mean by "craft the query". Is it not possible to broadcast all subnets (arp -a) in my case? – Gobliins Oct 12 '12 at 08:19
@Gobliins: What would go in the "target protocol address" field? – David Schwartz Oct 12 '12 at 08:26
until now i would leave it blank. But maybe insert something like "255.255.255.255" – Gobliins Oct 12 '12 at 08:35
Then no device would respond. – David Schwartz Oct 12 '12 at 09:01
you mean i would need information about the subnets? i.e. 192.168.x.x or 169.254.x.x – Gobliins Oct 12 '12 at 09:28
You would need some way to craft a query the device would respond to. Sending broadcast ARP queries for all 3 billion valid IP addresses is probably not practical. Knowing at least the subnets in use would help a lot. – David Schwartz Oct 12 '12 at 09:29

arp across subnets which devices can be detected?

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