Network Problems

I'm not sure what I'm asking is relevant to this site. I have multiple subnets of groups of plcs that I have segregated. I'd like to pull information out of all of them to a central point for data collection. I'm having a hard time figuring out how to setup the ip address and subnet mask of the data collection point to be able to talk to all of the different subnets. I've heard that using a router is a way to talk to all the different subnets. My question is that has anyone run into this problem and, if so, could you point me in the right direction about how to handle this problem?

Thanks

The Subnet Mask is a value that determines what bits in the IP Addresses have to match in order to communicate. The default "255.255.255.0" means that the first three octets have to match, the last one can be anything.

What subnets are you using?

If for example, you have something like:
Group A
192.168.1.100
192.168.1.101
192.168.1.102
192.168.1.103

and

Group B
192.168.2.100
192.168.2.101
192.168.2.102
192.168.2.103

All you really need to do is make the mask 255.255.0.0 and now they can communicate to each other, and the central data collecting PLC that you want to put on 192.168.3.100 with subnet mask of 255.255.0.0 should be able to access data from them as well.

This is how I understand it. A router can handle different subnets while not having to worry about changing your Mask. I don't think it's recommended to unmask the first two octets...

Hi everyone:
As Tharon says, if you have "different networks", you must use a router or switch layer 3. Usually, PLC networks are from same segment, therefore you don´t need a router.
Please, remember if you want to use a router, you need a small configuration for make a routes between one group of PLC to each other.

Regards,

MarcoSoft

Keep in mind the less restrictive your subnet mask, the more IPs that a broadcast will hit.
255.255.255.0 <- 254 addresses per broadcast, 255.255.0.0 <- 65534 addresses per broadcast, 255.0.0.0 <- 16777214 addresses per broadcast...

How many Subnets are we talking about, and what are their IP Ranges and Masks?

Stu....

I sent an email just yesterday to our IT department asking them about this very thing. They just told me I was stuck with the 0-255. Present setup is 10.8.0.XXX and 255.255.255.0
So does this mean we can start new IP Addresses at 10.8.1.XXX and if I set my laptop to 10.8.0.xxx, 255.255.0.0. I can see both?

Thanks
George

George Graziano said:

I sent an email just yesterday to our IT department asking them about this very thing. They just told me I was stuck with the 0-255. Present setup is 10.8.0.XXX and 255.255.255.0
So does this mean we can start new IP Addresses at 10.8.1.XXX and if I set my laptop to 10.8.0.xxx, 255.255.0.0. I can see both?

Thanks
George

Click to expand...

You are kind of hijacking Beanpole's thread, but at least it is related. The short answer is no. The Netmasks for all the devices on both Subnets would have to match for this to work.
With a Netmask of 255.255.255.0, the first three Octets have to match for the device to reply.

http://en.wikipedia.org/wiki/Subnetwork

Stu....

Sorry for the delay and thanks for all of the responses. I have about 10 different subnets. The reason for so many is for the very same reason dravik said earlier. We didn't see it in the beginning because there weren't a lot of plcs. But the guy before me set up all the plcs for a class b subnet so that he could collect data from them. As i've added to the network and it started to grow i'm starting to see my traffic going up. I talked to a few programming friends and they said its probably because AB plcs like to broadcast when they send data. I also talked to rockwell and a tech there informed me that i would indeed have to have either a layer 3 switch or a router that's configured.

Another option i just had was that i'm only collecting data from one plc per subnet. Couldn't i set up that one plc as a class b and all the others as a class c and be okay with no router? This is confusing but for example:

Subnet 1 is 10.0.1.0-254 with a mask of 255.255.255.0
Data collecting plc is set up for 10.0.1.x with a mask of 255.255.0.0

Subnet 2 is 10.0.2.0-254 with a mask of 255.255.255.0
Data collecting plc is set up for 10.0.2.x with a mask of 255.255.0.0.

The data collection computer would then be setup for a mask of 255.255.0.0 and then be able only to talk to the data plcs. But the data plcs can then talk to all the plcs on it's little subnet. Would that work?

Would that work ?

Well I think it will, but I'd like to hear a more expert view...

Couldn't you change the subnet mask of the PLCs in question, so they can communicate to any of the data collecting PLCs? (And maybe in return you could limit it to a single collector?).

Or is changing the PLC network settings not an option?

I'm not well versed in large scale networks like this, I understand what was posted earlier about broadcasting. If you opened these subnet masks up, would all the broadcasts swamp the network and cause issues?

You could even open it just a little, just the first two bits of the octet.
255.255.252.0
11111111.11111111.11111100.00000000

That way they can communicate to 10.0.0.XXX, 10.0.1.XXX, 10.0.2.XXX and 10.0.3.XXX, and wouldn't add as much in term of broadcasting. Only 255x4 instead of 255x255

Right, using a /22 instead of a /24 would give you
10.0.0.0 - 10.0.3.254

"The data collection computer would then be setup for a mask of 255.255.0.0 and then be able only to talk to the data plcs. But the data plcs can then talk to all the plcs on it's little subnet. Would that work?"

Not really, For the systems on a different subnet than the data collection pc, Their more restrictive subnet mask will cause packet loss. Basically, the data collection PC would be able to see anything they sent, but the PLCs would ignore all requests(since they are originating from a subnet that is filtered out).

That is an idea that i'm now considering Tharon. I've already thought about modifying the subnet mask to a unique class instead of strictly class b or c. It would have to be something like 255.255.224.0 to allow for expandability. Traffic is an issue though. The switches (i have managed and unmanaged) try to do a good job with keeping the traffic down. I was just trying to make it a little more reliable for the future by preventing someone from addressing across lines accidentally. If i had them restricted to their own subnets then i could prevent accidental traffic on our roof fiber network.

I've also talked with one of my programming buddies told me about the auto speed detect problem with allen bradley. All of my plcs are also setup for auto speed detect on the ethernet ports. He told me to set them all up with 100Mb/s to force it to run as fast as possible. He said that this issue has caused him excessive ethernet traffic in the past.

I see what you are saying Dravik. The scenario is for example 3 plcs. A master plc (to collect or watch the line for status updates) setup for 10.0.1.100 subnet 255.255.0.0, and two device plcs 10.0.1.101 and 10.0.1.102 with subnet 255.255.255.0.

The data collection pc is setup for 10.0.15.100 with a subnet of 255.255.0.0. I only want to gather data from the master plc anyway. The pc could care less what the other plcs are doin. The only things i'm worried about is can i get data from the master plc to the pc. Also can the master plc talk back and forth to the device plcs with this whole subnet setup?

I think that makes my whole scenario a little more clearer. It's a unique solution that if i could change easily i would but there are just too many devices setup already. i can change subnets pretty easily but to chane the ip addresses would take a very long time. We have drives, plc's, pv's and pv+'s all addressed to talk to each other. As long as i just change the subnets and do it properly everyone should stay happy right?

Thanks for all the help and responses so far.

Hopefully this weekend i'm going to run some testing on 3 plcs that aren't hooked to anything yet and see if the scheme would work with Tharon's idea of a unique subnet. Also i'll try some other things to see if i can get some good connections too. To recreate a bunch of traffic i'll set msg blocks up to sending out every 150 ms or so between all three and see if they are all being sent properly. I'll repost with my results on Monday.

Let me know if anyone has any other ideas. I really appreciate the help.

In the example you used above, yes that would work. However, please use the most restrictive subnet mask you can. 255.255.0.0 would be better served as 255.255.240.0(this would give you 10.0.0.0 - 10.0.15.254)