https://docs.netgate.com[...]
Normally each interface on the pfSense® firewall represents its own broadcast domain with a unique IP subnet. In some circumstances it is desirable or necessary to combine multiple interfaces onto a single broadcast domain, where
two ports on the firewall will act as if they are on the same switch, except traffic between the interfaces can be controlled with firewall rules. Typically this is done so multiple interfaces will act as though they are on the same flat network using the same IP subnet and so that clients all share broadcast and multicast traffic.
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