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Armadillo852

Ethernet MAC Address

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Does anybody know of a way to be able to get an Ethernet MAC address from an EN2T Ethernet card without pulling the card out, or shutting down production?

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Sure, I would suggest getting a hold of SoftPerfect's Network Scanner utility (netscan). It and wireshark are two network utilities I would suggest having. Both are freeware or shareware. Netscan will search out all IP's on the network in a range you select to let you know what's out there. It also returns the MAC address associated with the IP. This is their website: http://www.softperfect.com/

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Thanks for the quick response. Kind of funny, but wireshark looks too complicated for me to use.

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1. Open up a command prompt on a computer connected to the network. 2. PING the 1756-EN2T module "PING 192.168.1.100" (with the module's IP address, of course) 3. Type "ARP -A", hit return. The results of the ARP (Address Resolution Protocol) will include a list entry with the IP address of the 1756-EN2T and its MAC ID.
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I tried that and it was weird, it would give a response when pinged. But that IP did not show up when I tried the arp -a. Maybe because it is going threw a managed switch? I saw that article on AB's knowledge base.

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Try this: Open a tab in your favorite web browser and type in the IP address in the address bar. Mine show the MAC ID on the built in web pages but I only have two Logix PLCs and they're both pretty new firmware versions.

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That worked wonderfully. Not sure why I didn't think of that.

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Yeah wireshark is a little involved, but netscan is all you need for getting all the mac id's. Using the browser to access the built in web page is pretty simple. Gotta love the simple solutions... ARP -A works well too. For some reason I can just never remember that. Edited by PLCMentor.com

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I consider myself a networking novice, but I found wireshark quite easy to use. I was hesitant to try it out for a long time, but now I feel silly for not downloading it sooner. It helped us figure out a really weird problem with IGMP snooping.

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This *has* to work. Something is very wrong if it does not. If you are going through a router it is possible the MAC address may not be the one you are looking for, but something has to be there in that table. After you check the web page, the entry should be in the same table.

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What happens is we map one IP address to a totally different IP address. Physical address of Ethernet Card in rack is 153.0.0.147. The IP address we map it to (Which is what we use to connect to the outside world) is 192.168.50.147. This might be why I cannot see it on the arp -a list. Eitherway, I have them using Google chrome web browser.

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You are on to something here and no, it definitely does not "have" to work. We're talking about a layer 2 system here (Ethernet) and a layer 3 system (IP network). The original network is Ethernet. The IP network is an "overlay" over the top of the underlying Ethernet network. Understanding how the routing works is key to understanding why ARP -A may or may not work, and also generally helps to understand why MAC addresses matter, and why routing may or may not work at all when you expect it to. Each device in your Ethernet network has a MAC address. But you typically work with IP packets. ARP (Address Resolution Protocol) is an Ethernet protocol which is used to map IP addresses to MAC addresses. Inside the device, it has an ARP table, which is a cache of the IP/MAC map. There are also some preconfigured addresses in there and some "static" (manually programmed) MAC addresses if you do this (most people never use this, but there are some special preprogrammed entries). Whenever a packet is ready to be sent out across the Ethernet, the corresponding "link layer protocol" (software responsible for routing the packet over Ethernet) first consults the local ARP cache to see if it can directly find a corresponding MAC address. If it can't find one, then it sends out a broadcast packet using the ARP protocol. The local Ethernet device with the correct IP address responds, and then the device stores this new IP/MAC address mapping in it's ARP table. If the first request fails, then the IP routing algorithm takes over. Instead of sending the packet directly to the corresponding MAC address, it takes the "gateway" or "router" address which is preprogrammed into your IP settings and attempts to route across the Ethernet again, only this time to the gateway/router address. If after the second trip through ARP it comes up with "unknown", then you get an error. However, if it does learn the MAC address of the gateway, it sends the IP packet out across the Ethernet to the gateway. The gateway in turn uses IP/ARP routing protocols to send the packet on it's way out to a totally different network...hence the term "internet" or "internetwork". IP routers in turn use their own set of broadcast/search protocols (such as BGP) to hunt for a corresponding route across an internet, and respond if the device can't be found using ICMP. And again, there are some preprogrammed special entries...10.x.x.x and 192.168.x.x are "nonroutable" (should never see this address outside of an intranet...all internet routers are programmed to reject/ignore these addresses). Now...getting back to why ARP -A doesn't work. In DOS (or Windows), the command "ARP -A" will print out the local ARP table for you. So if you recently executed a PING command to an IP address, then ARP -A will show you the IP address of the device except in four cases (with AB PLC's). First, if you don't enter the command right away and the ARP cache forgets the address, you'll never see it. Most ARP caches are good for around 50 seconds. Second, if you have a manually programmed ARP address and it's wrong, ARP has no way to deal with this, but this is rare (hardly anyone knows about manually entering ARP addresses). Third, if the device isn't on the local network (and went to the gateway instead), you will not find a MAC address, only the gateway address (it got routed over IP, not Ethernet). Finally, if you routed your packet across a CIP gateway such as routing in one ENBT card, across a 1756 chassis, and then out another one, you'll never see anything but the first ENBT card with ARP. Probably the three most reliable methods to get the address are either to browse the ENBT card's web server page with your browser (if you can get there, still fails if you routed over CIP), or else to right click on it from Logix 5000 and click "properties" (works in all cases), or else using RS-Networx (if you have it).

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Your right - I didn't fully think through my comments before posting - I look up MAC addresses all the time, and they are *always* there... but I don't route through to my PLCs so for me, I expect to see them in my arp table - always, when looking. Others with more sophisticated networks may not be in the same situation.

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OK here is one reason the ARP did not work: I did not ping the IP address I wanted first. Secondly, I pinged every address I wanted to see the MAC address for. Third, I am sure I waited more than 50 seconds before requesting the ARP. Now, one other thing. If I right click on the properties of the EN2T module, I cannot see anyplace where the MAC address is. Not sure if there is on the ENBT or not. I can see the IP address, obviously. I did use the web browser and was able to get all the MAC addresses without having to shut production down. The biggest issues we have is our office is in Chicago area, and we connect via VPN/VNC to our PLC's. Thanks for the explanation guys and all the help. As usual, this forum did not let me down. Corey

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I'm dealing with this issue right now and sure would be nice if RA would put a MAC ID Sticker on the unit like other computer hardware vendors.

I'm struggling with a new laptop and super tight security {carbon black, no admin user account, etc}  as I try to setup a USB to Ethernet Adapter and get a look at this modules web page and/or ARP -a results.

Oops - Wait it was buried beside the thumbwheel switches.  SO tiny and just the address not an indicator it was MAC ID.

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It would also be nice if the MAC ID label was visible while the module is installed instead of on the side where you have to pull it out of the chassis... I've encountered that issue a bunch.

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On 1/4/2011 at 7:48 AM, Armadillo852 said:

Thanks for the quick response. Kind of funny, but wireshark looks too complicated for me to use.

Youtube!.  Wireshark is actually very easy and powerful to use 

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if you are at least with in the same subnet, you should also see the MAC in BootP/DHCP tool.  Additionally, your should be able to access the internal webpage of the ENBT via a browser and see the MAC there as well.  If I'm repeating, I apologize, didn't read entire thread.

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