Subnet mask causing connection issues

[Poei]

I am working with a 1768-ENBT and I was able to connect to it through my laptop. My laptop IP is 192.168.1.10 subnet 255.255.255.0 and the PLC was IP address 192.168.1.100 subnet 255.255.255.0. Set up like this the connection worked fine.

After changing the PLC subnet to 255.255.255.224 I can no longer connect to it. Even after changing my laptop to the same subnet, I cannot connect. I can't see it in RsLinx and I can't ping it. I tried resetting the PLC back to factory default but I can't get that to work. How can I change the subnet back to what it was if I can't connect to it?

 

[TheWaterboy]

An IP from 1 to 30 should connect with that mask. Is there a router between you and it which might require a Gateway entry?
Use a cheap switch and connect to it directly to change it.

 

[jstolaruk]

See if you can ping the ENBT at 192.168.1.4 (long shot). Once you change the mask back, 100 will be the address again.

All else fails, you need to access through another module's backplane connection and use RSLINX to make the change.

 

[Poei]

reply

There is no router or switch, I am plugged in directly to the PLC from my laptop. I tried pinging 192.168.1.4 but that didn't work.

 

[jschulze]

If you didn't change either the ENBT's or your laptop's IP address when changed the subnet masks for both, this will of course not work because you have now put them on different subnets. You will need to reconfigure your laptop's IP to a compatible IP address on the same subnet as the ENBT. Assuming the ENBT is still configured for 192.168.1.100, the compatible subnet range would be 192.168.1.97 - 192.168.1.126.


This online subnet calculator is useful for the above calculations:
https://www.calculator.net/ip-subnet-calculator.html

 

[jstolaruk]

If you didn't change either the ENBT's or your laptop's IP address when changed the subnet masks for both, this will of course not work because you have now put them on different subnets. You will need to reconfigure your laptop's IP to a compatible IP address on the same subnet as the ENBT. Assuming the ENBT is still configured for 192.168.1.100, the compatible subnet range would be 192.168.1.97 - 192.168.1.126.


This online subnet calculator is useful for the above calculations:
https://www.calculator.net/ip-subnet-calculator.html

I believe thats the OP's problem, he can't get to the subnet to change it.

Only way I know for sure is to somehow get on the backplane and change it from RSLinx. Any module that has a USB faceplate port or ethernet port can bu used to drill down to another module on the backplane.

 

[jschulze]

You will need to reconfigure your laptop's IP to a compatible IP address on the same subnet as the ENBT.

The OP was able to change the subnet mask of his/her laptop, so he/she should be able to change the IP address of the laptop as well.

 

[jstolaruk]

The OP was able to change the subnet mask of his/her laptop, so he/she should be able to change the IP address of the laptop as well.

If I'm reading his posts correctly, he's got the ENBT address at 192.168.1.100 and the mask set to 255.255.255.224. What can he set is laptop to so that he can communicate with it again?

 

[Poei]

If you didn't change either the ENBT's or your laptop's IP address when changed the subnet masks for both, this will of course not work because you have now put them on different subnets. You will need to reconfigure your laptop's IP to a compatible IP address on the same subnet as the ENBT. Assuming the ENBT is still configured for 192.168.1.100, the compatible subnet range would be 192.168.1.97 - 192.168.1.126.


This online subnet calculator is useful for the above calculations:
https://www.calculator.net/ip-subnet-calculator.html

This worked! I changed my laptop IP to 192.168.1.99 and I was able to connect with the ENBT and change the subnet back to 255.255.255.0. I changed my laptop IP and subnet back to what it was and I can see and connect with it in rslinx. Thank you!!

 

[jstolaruk]

I know I learned something from that (actually a long time ago), don't mess with communication settings unless you've got a back door to get in and return it (ie backplane connection in this case). Glad to hear you've gotten past it.

 

[jschulze]

If I'm reading his posts correctly, he's got the ENBT address at 192.168.1.100 and the mask set to 255.255.255.224. What can he set is laptop to so that he can communicate with it again?

The OP set his/her laptop's IP address and subnet mask to 192.168.1.99 and 255.255.255.224, respectively, to be compatible with the ENBT at IP address 192.168.1.100 and subnet mask 255.255.255.224.

 

[drbitboy]

I am working with a 1768-ENBT and I was able to connect to it through my laptop. My laptop IP is 192.168.1.10 subnet 255.255.255.0 and the PLC was IP address 192.168.1.100 subnet 255.255.255.0. Set up like this the connection worked fine.

After changing the PLC subnet to 255.255.255.224 I can no longer connect to it. Even after changing my laptop to the same subnet [nope], I cannot connect. I can't see it in RsLinx and I can't ping it. I tried resetting the PLC back to factory default but I can't get that to work. How can I change the subnet back to what it was if I can't connect to it?

255.255.255.224 (a.k.a. /27) and 255.255.255.0 (a.k.a. /24) are not subnets, they are subnet masks (a.k.a. netmasks).

The subnet of an IP configuration is the bitwise-ANDed combination of that configuration's IP address and that configuration's subnet mask.

For subnet mask 255.255.255.224, the subnet

• of 192.168.1.10 with subnet mask 255.255.255.224 is 192.168.1.0

• i.e. 0 = 224 & 10 = (128 + 64 + 32) & (8 + 2)

• of 192.168.1.100 with subnet mask 255.255.255.224 is 192.168.1.96

• i.e. 96 = 224 & 100 = (128 + 64 + 32) & (64 + 32 + 4)

• of 192.168.1.99 with subnet mask 255.255.255.224 is 192.168.1.96

• i.e. 96 = 224 & 99 = (128 + 64 + 32) & (64 + 32 + 2 + 1)

For subnet mask 255.255.255.0, the subnet

• of 192.168.1.10 with subnet mask 255.255.255.0 is 192.168.1.0

• i.e. 0 = 0 & 10 = () & (8 + 2)

• of 192.168.1.100 with subnet mask 255.255.255.0 is 192.168.1.0

• i.e. 0 = 0 & 100 = () & (64 + 32 + 4)

 

[lfe]

A 192.168.1.0 subnet with 255.255.255.224 mask is a very small net, available IPs are 192.168.1.1 to 192.168.1.30 therefore IP 192.168.1.100 It is not accessible