Networks·41,395 discussions

Subnet change to expand IP range

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Tags: Networking, Security, Windows

Bare with me here, I have taken one networking class but it really confused me so I need a little more help here.

So when the plant opened 10 years ago, the contractors setup the subnet to 255.255.255.0. With this we only had probably 252 available IP address.

PLC’s are all Static IP addressing, so they are set and we used managed Hirschman switches to create a ring network for each building.

We are trying to expand our IP addresses due to expansion and due to the fact that now we probably have a few open spots here and there but that would not be ideal to use for expansion since it wouldnt be an organized network.

IT department is partly involved, they know that we have to expand and they suggested going to 255.255.252.0

Its a great idea, our issue is…
The plant basically runs 24/7 and cant stop the flow coming in for too long.
We have multiple ring topologies around the plant connected to a main ring topology. This was done for redundancy and cost measures.

What were hoping is that when we change the PLC to the new subnet that it connects to the switch without issues when we change the switches subnet as well.

Do you have anything to add to this? I need some networking gurus to help me out. Do you know if theres a way to change the subnet quickly or like all at once?
Do you think we should start with the outermost ring and work our way inwards?
What else should we be worried about?
Will a change in subnet bring the network completely down?

Topic

[rproffitt]

Reply To: Subnet change to expand IP range

In reply to Subnet change to expand IP range

But for non-Internet IP setups I usually go for 255.255.0.0 so I don’t have to revisit this later. Today’s switches send traffic where it needs to go so the old school LAN segmentation is usually never a problem.

You didn’t mention what DHCP server you are using so for me I start the change there and let it propagate.

Finally I don’t have the change all devices. I only need to change the (is it a) PC that talks to the devices and then the new devices. The old devices most likely don’t need the netmask change unless we are changing the IP address of the usual PC that monitors the devices.

In other words, there is a chance there is actually very little work to be done.

• This reply was modified 8 months, 3 weeks ago by rproffitt.
• This reply was modified 8 months, 3 weeks ago by rproffitt.

Reply To: Subnet change to expand IP range

In reply to Reply To: Subnet change to expand IP range

So ideally we would go with 255.255.0.0. Our network is mainly managed by our IT department. They dont really like the idea of going to 255.255.0.0 and are the ones who gave us the 255.255.252.0 which will give us about 1000 IP addresses.
But supposedly all the devices within the plant is off network. We have a blue ethernet port only for SCADA items and yellow ethernet port for internet stations.

The closed network we have is not DHCP, its a static IP addressing because our PLC’s do not allow DHCP. They all need assigned IP addresses, so does other IP linked devices.

If we were to change the subnet to the new 255.255.252.0 on the switches with a static IP addressing protocol would the device go offline?

No. Unless.

In reply to Reply To: Subnet change to expand IP range

The only reason I can see for the devices with 255.255.255.0 to go offline is if the PC or monitoring system is moved to a non 255.255.255.0 IP Address.

Anyhow, I would in the case where the monitoring systems are staying with their assigned IP, change the monitoring PC netmask and deploy the new devices as required.

I see no reason to change old working devices. Do you?

Thank you!

In reply to No. Unless.

Okay that makes sense. Thank you.

The old devices will all stay with the same IP and the new devices will be on the new range.

Thank you for the advice! I appreciate it.

[Author]

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