While the title states PLC ethernet network addresses, this post applies to any device on ethernet. With ethernet growing rapidly in industrial automation, and many if not most control engineers not having a mainstream IT networking background, this is a primer on the topic of IP addressing and subnets.

Ethernet connectivity is included on many if not most newer PLC’s and industrial automation devices.  As such, basic knowledge of Ethernet networks is a requirement for control engineers and automation personnel. The topic has come up several times over the last few months on PLCTalk. This weeks pick is a post by PLCTalk longtime user Operaghost. It is  from March 2016 and covers some important notes about networks and subnetworks ( subnets).

The post has been published here with permission from Operaghost. Operaghost’s text is in blue. Some parts are expanded further with the diagrams below.

Subnetworking is used to define a specific division out of a larger network. The ‘Subnet mask’ is applied to an IP address to define the ‘network prefix’ for the subnet. The rest of the available bits in an address becomes host addressable addresses within the subnet. More details here

The original question by WillM is as follows ( in green):

Simple question but I want to be certain. Lets say you have 2 separate PLCs on a network with the following configuration…

PLC 1 
192.168.1.1
255.255.255.0

PLC 2
192.168.1.2
255.255.0.0

If i set my PC up with the following…
192.168.1.X
255.255.255.0
or
192.168.1.X
255.255.0.0

Will I be able to access both PLCs without having to change the subnet?

I’ve tested it and it works. I’ve read up on subnets and in my head it works but someone has told me there may be issues. My understanding is that both IP addresses are within the smaller Subnet of 255.255.255.0 so if my laptop was set to 255.255.0.0 it would be accessible.

Am i missing something or is it ok?

There were several responses in between, and then Operaghost’s  descriptive response ( in blue below):

Ok, there is some misinformation going on here. Yes, what you described will work.

 But the mixed subnet masks is usually not a good idea just from a organization aspect. But it is certainly allowed.

So this may be more background than you wanted, but I wanted to set the record straight.

 

Breakdown of IP Address Ranges for Networks

Class A addresses range from 1.0.0.0 – 127.255.255.255. So for example, 10.0.0.1 starts with a 10 so it is a Class A address. Class A addresses use 8 bits to represent the network address and 24 bits to represent the individual hosts. So with Class A I could have 128 individual networks with each network containing over 2 billion host devices. 

The bit level representation here is in reference to the 4 octets being made up of one byte each. The entire IP address breaks down into 32 bits ( diagram above).

Class B addresses range from 128.0.0.0 – 191.255.255.255. So for example, 172.16.0.1 starts with a 172 so it is a Class B address. Class B addresses use 16 bits to represent the network address and 16 bits to represent the individual hosts. So with Class B I could have 16,384 individual networks with each network containing over 65,000 host devices.

Class C addresses range from 192.0.0.0 – 223.255.255.255. So for example, 201.35.0.1 starts with a 201 so it is a Class C address. Class C addresses use 24 bits to represent the network address and 8 bits to represent the individual hosts. So with Class C I could have over 2 million individual networks with each network containing over 256 (actually 254) host devices.

We won’t get into Class D or E.

Class A Typically use a subnet mask of 255.0.0.0
255 = 8 consecutive 1 bits followed by 24 consecutive 0 bits is often known as “/8”

Class B Typically use a subnet mask of 255.255.0.0
255.255 = 16 consecutive 1 bits followed by 16 consecutive 0 bits is often known as “/16”

Class C Typically use a subnet mask of 255.255.255.0
255.255.255 = 24 consecutive 1 bits followed by 8 consecutive 0 bits is often known as “/24”

This is what is known as CLASSFUL Addressing. This basically meant if 192.168.1.1 was your IP address, then 255.255.255.0 was your subnet mask. Automatic, no ifs ands or buts.

Classless and Classful Addressing

 

But once IP addresses started to dwindle in availability, we saw how wasteful this addressing scheme really can be. So today, it is not required and most networking people have abandoned it. We can instead use CLASSLESS addressing. But, you don’t really want to mix methods as it can be very confusing and frustrating.

So with CLASSLESS it is now commonplace to see a mix of address classes and subnets. So your example of 192.168.1.2 with a subnet of 255.255.0.0 is a perfectly acceptable CLASSLESS address. So the whole idea of Class A, B, and C are not terribly relevant with the classless addressing scheme.

Your address of 192.168.1.X with a subnet of 255.255.0.0 is simply saying that this is a network identified as 192.168.0.0. The first IP address in the range is known as the network address. The mask is identifying that the last address on your network would be 192.168.255.255. Now the very first address and the very last are reserved so (256 x 256) – 2 = 65,534 devices could all be on the same network. That is potentially a very large network. One device sending a broadcast message would go to all of those devices.

Your address of 192.168.1.X with a subnet of 255.255.255.0 is saying that this is a network identified as 192.168.1.0. The mask is identifying that the last address on your network would be 192.168.1.255. So 254 devices. That is a much smaller network. One device sending a broadcast message would be contained to only go to those devices. 192.168.0.0 would be a completely separate network as would 192.168.2.0 and so on.

But what if your network didn’t require 250+ devices. What if it only needed to handle 10 devices. If we assigned 192.168.1.x with a subnet of 255.255.255.0 we would potentially be wasting 240+ IP addresses that we could use elsewhere.

Subnet Mask

So the mask of 255.255.255.0 is typically referred to as “/24” as it represents 24 consecutive 1 bits representing the network. The remaining 8 zero bits is where we get how many hosts we can have. But if we change how many network bits are used then we can affect how many host bits are available. So are not stuck with 256 or 65,000 as our only choices.

 

255.255.252.0 or /22 = 1024 (-2 reserved) hosts per network
255.255.254.0 or /23 = 512 (-2 reserved) hosts per network
255.255.255.0 or /24 = 256 (-2 reserved) hosts per network
255.255.255.128 or /25 = 128 (-2 reserved) hosts per network
255.255.255.192 or /26 = 64 (-2 reserved) hosts per network
255.255.255.224 or /27 = 32 (-2 reserved) hosts per network
255.255.255.240 or /28 = 16 (-2 reserved) hosts per network
255.255.255.248 or /29 = 8 (-2 reserved) hosts per network
255.255.255.252 or /30 = 4 (-2 reserved) hosts per network

 

So using /28 as our mask we can “sub-network” a typical range into many separate networks.

We could have the network start at 192.168.1.0 and end at 192.168.1.15
We could have another network start at 192.168.1.16 and end at 192.168.1.31
We could have another network start at 192.168.1.32 and end at 192.168.1.47
etc, etc…..

This idea of sub-netting is much more common in the IT world where they have a set number of IP addresses available and they cannot be wasted through a wasteful addressing scheme. In the controls world, we typically have had relatively small networks that have been isolated from the enterprise so our addressing methods didn’t really matter all that much. Today though we are seeing our control networks connecting to the enterprise network and knowing these addressing schemes becomes quite important.

So I have gone on much longer than I intended and I am sure I lost people along the way. But I wanted to make sure we understand that classless addressing is here to stay which makes the whole idea of Class A, B, and C mostly irrelevant.

OG

The entire thread on PLCTalk can be read here.