FIREWALL RULES ON LHG5

Hi, I’m not good at English but I’ll do what I can to explain my situation.

1- I have my lhg5 connected to a local network from which only the users of subnet 88.6.6.0/24 must have access to it. I need a rule for that.
2- sometimes I see that the RX traffic of the ethernet increases to 30Mb when it should be on 2Mb, which I think is Broadcast traffic by a defective switch.

can someone please tell me what basic rules I could implement in the firewall.
Thanks a lot