Confused about subnet conflict on machine network...?

So I thought I understood networking better than I apparently do... can someone explain this to me:

We have a machine with an NTRON 700 switch and star topology. Connected to this switch are an AB CompactLogix PLC, several remote IO blocks, an HMI, and several servo drives (Kinetix).
All devices are using IP addresses of the form 10.107.8.XXX
The subnet on the PLC, the HMI and one of the remote IO blocks uses a 255.255.0.0 subnet. The subnet on the servos and remaining remote IO is 255.255.255.0

In RSLinx I can only 'see' and configure the devices on the same subnet as my laptop's NIC card (so depending on which subnet I have my NIC set to, I only get half the devices). Yet, when I connect to the PLC and go online with it the PLC is having no issues communicating with all devices and the machine seems to be running fine.

How is this??? My expectation would be that the PLC (with subnet 255.255.0.0) would not be able to see or communicate with any of the devices on the 255.255.255.0 subnet (just as RSLinx on my PC cannot when I am set to a 255.255.0.0 subnet). Obviously this is not the case, but I'm having a hard time understanding why. My only guess is it has to do with the Layer-2 vs. Layer-3 (MAC vs. IP) nature of the connection through the switch, but I'm having a hard time understanding the specifics of what is going on...

Enlighten me!

Thank you
-Z

I would take a look at this recent (and active) thread: http://www.plctalk.net/qanda/showthread.php?t=101953

There's a little bit of what you're talking about in there. It seems like some devices require matching subnets and some require identical subnets in order to communicate.

Technically they are all on the same network 10.107.8.0. The devices with the 255.255.0.0 mask see the network as potentially containing 65,000 devices. The other devices see a network that can only handle 256 devices.

I suspect you have an issue though with Broadcast traffic though. The last IP address is reserved for broadcasts. So the devices with 255.255.255.0 are using 10.107.8.255 when they need to broadcast but the other devices are broadcasting at 10.107.255.255. So they are not seeing each other's broadcast traffic.

Mixing subnet masks while allowed is generally not recommended as it leads to exactly this kind of confusion.

As for your computer......In RSLinx Classic are you using the EtherNet/IP driver or the Ethernet Devices driver?

I don't recall exactly but I believe the EtherNet/IP driver will only browse it's own network (subnet) or a specific network that you identify.

The Ethernet Devices driver requires you to manually enter the IP address but it can access devices on other networks.

I really don't use the EtherNet/IP Driver that often so I might be mistaken. I'm sure someone can chip in to tell us one way or the other.

OG

Operaghost said:

I suspect you have an issue though with Broadcast traffic though. The last IP address is reserved for broadcasts. So the devices with 255.255.255.0 are using 10.107.8.255 when they need to broadcast but the other devices are broadcasting at 10.107.255.255. So they are not seeing each other's broadcast traffic.

Click to expand...

Good point about the broadcast traffic, I didn't see that one straight of the bat.

However would not broadcasta be received by everyone since a switch would forward it to all ports? DHCP broadcasts but all PLCs have static IP address so no DHCP requests. And an ARP broadcast for MAC address wouldn't that also be answered by all?

You are referring to Layer 2 broadcasts where I was referring to Layer 3 broadcasts. Broadcast messages to everyone that is part of my network versus everyone connected to all inter-connected switches.

OG

Also the N-Tron 700 is a managed Switch. You may want to look at how it is being managed.

Operaghost gave the answer, but I would suggest grabbing wireshark and putting it on your PC and have a look for yourself at what traffic is being sent out and then you will have a much better understanding of why this is occuring.

A network with a subnet mask of 255.255.255.0 is a class C network, and can have 256 addresses (ex. 192.168.1.0-255)

A network with a subnet mask of 255.255.0.0 is a class B network, and can have 65,536 addresses (ex. 192.168.0-255.0-255).

This is why your devices configured for class B can communicate to the devices configured for class C. As long as the first 2 octets of the I.P. address are the same; the class C is part of the class B network. Inversely, the devices configured as class C, cannot communicate outside the 256 addresses (192.168.1.0-255) without routing.

iraiam said:

A network with a subnet mask of 255.255.255.0 is a class C network, and can have 256 addresses (ex. 192.168.1.0-255)

A network with a subnet mask of 255.255.0.0 is a class B network, and can have 65,536 addresses (ex. 192.168.0-255.0-255).

Click to expand...

Well, I have never heard anyone ever call an IP address that started 192.x.x.x a Class B address before.

So if I look at my IP address currently set to 10.45.1.3 with a subnet mask of 255.255.255.0 are you saying that is a Class C address? Of course not. Is it Class A? No, it isn't.

Look at the link in post #2 and read my post there. You will see a fairly thorough description of Class A/B/C and the use of CLASSLESS addressing. The IP address I specified above is an example of a perfectly valid CLASSLESS address.

Careful with blanket statements.

OG

Class A addresses start with IP address 0 to 127
Class B addresses from 128 to 191.
Class C addresses from 192 to 223.

https://en.wikipedia.org/wiki/IPv4_subnetting_reference


Ranges available for private (non-internet) networks.
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

https://en.wikipedia.org/wiki/Private_network


Classful networking
https://en.wikipedia.org/wiki/Classful_network

Classless networking
https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

.