Network question from PM

[Michael Walsh 268]

From a PM I received today:

I have a Omron CP1L-E with IP: 192.168.3.55 mask 255.255.248.0 GW 192.168.1.95

if the pc ip address is: 192.168.3.50 MASK 255.255.248.0 GW 192.168.1.95 PING WORK

if the pc ip address is: 192.168.1.96 MASK 255.255.248.0 GW 192.168.1.95 PING does not work

does the plc respond to ping only if the pc has an ip of the same class?
with this mask (255.255.248.0) plc and pc are in the same subnet  in both cases.

you have an explanation?

Thanks

[panic mode 246]

nodes need to be in same subnet in order to communicate (that includes getting response from PING)

subnet is defined by subnet mask. essentially it splits IP address into two fields: LAN address and NODE address

 

for nodes to be part of same subnet:

1. Subnet Mask must be valid (and your is)

2. SubnetMask must be the same on all nodes (and yours is)

3. LAN address must be the same (and yours is)

4. NODE address must be unique (and your is)

so this should work in both cases.

 

mask 255.255.248.0 in binary form looks like

11111111.11111111.11111000.00000000

another way to represent this is /21 because 21 bits on the left are set (all "1" must be on the left, all "0" must be on the right)

192.168.3.55  in binary looks like:

11000000.10101000.00000011.00110111

while 

the 21 bits on the left are the LAN address 11000000.10101000.00000000.00000000

remaining bits are NODE address is  00000000.00000000.00000011.00110111

in case of  192.168.1.96 one gets same LAN address and unique node address. this should work unless there is something else going on (GW configuration, routing or address overlap with other interfaces, one of the nodes does not support fully subnets and expects mask /24, etc.)

i would say set both nodes GW to 0.0.0.0 and try again.

and check for suspects using IPCONFIG/ALL and ROUTE PRINT.

 

[bambuli 0]

Okay,  thanks for your time. I'll update you as soon as I have done the suggested tests. My suspicion is that :   "one of the nodes (PLC) does not support fully subnets and expects mask /24 "

Edited 23 Jan 2020 by bambuli

[bambuli 0]

I have tried many configurations but it doesn't work. The ping responds only if the pc has an address with subnet /24
I accept any suggestions, thanks

(attached the current configuration that does not work)