What are the rules regarding the WAN vs. LAN addressing? If the first two octets are the same but the 3rd octet is different (ex: WAN - 10.0.3.x / LAN - 10.0.53.x), is that acceptable? Are these rules for subnet masks? Thanks,
The subnet address determines allowable ranges for communication.
10.0.3.x with a 255.255.255.0 subnet allows the network to communicate with 10.0.3.0 - 10.0.3.255
10.0.3.x with a 255.255.0.0 subnet allows the network to communicate with a 10.0.1.0 - 10.0.255.255
The LAN and WAN have to be on different subnets. Given your example above:
LAN 10.0.3.x with a 255.255.255.0 Subnet
WAN 10.0.53.x with a 255.255.255.0 subnet
will not cause a conflict.
Please see more information below regarding subnets