Cannot reach a client with ping

[bullfrog_au]

Hi all, first post and its an embarrassingly simple one.
I have just set up an OPNSense machine on physical hardware to act as a firewall/ router.

It is set up with the WAN interface connected to my existing LAN while I get it set up.
ISP Modem/Router -> Dumb Switch -> OPNSense Machine -> Dumb Switch -> Clients (2)

So far I have:

• Installed the OS
• Run the automatic interface assignment

DHCP is working correctly, and there are only the auto configured rules in the firewall.

Open DNS is disabled
Unbound DNS is enabled

All other settings are defaults.

• Both clients have internet access
• Both clients have OPNSense GUI access
• One client (192.168.x.100) can ping both the OPNSense machine (192.168.x.1) and the other client
• One client (192.168.x.101) can ping the OPNSense machine (192.168.x.1), but not the other client

I cannot understand why the client at .101 cannot be pinged from either the OPNSense machine or the client at .100.

I wont upload a bunch of screen shots yet, as I dont know what is helpful and what is not.
Thanks in advance for any help!

[tiermutter]

I assume the client .101 is located in the same subnet, hanging on the same switch as client .100. Then OPNsense is not involved in any traffic between them. Maybe .100 simply do not reply on ping...

[bullfrog_au]

I thought it would be embarrassingly simple...
Turned off the domain firewall in Windows security on the client @ .101 and hey presto.

Thanks tiermutter!!