A Bridge joins two or more interfaces to the same layer 2 (broadcast/collision domain), as if they were joined to the same switch.
Bridges between interfaces are listed and managed at Interfaces > (assign) on the Bridges tab.
When editing a bridge, Ctrl-Click to select the Member interfaces from the list of available interfaces and type a Description.
Advanced options may be used to enable features like Spanning Tree or other less common features of bridges.
Once a bridge has been created, it can be assigned under Interfaces > (assign) as any other interface.
A bridged interface can filter traffic without being involved in the IP layer of the connection. Bridge traffic is filtered on the member interfaces by default.
By default, traffic is filtered on the member interfaces and not on the bridge interface itself. This behavior may be changed by toggling the values of net.link.bridge.pfil_member and net.link.bridge.pfil_bridge under System > Advanced on the System Tunables tab. With them set at 0 and 1, respectively, then filtering would be performed on the bridge only.
Only one interface on a bridge should have an IP address! Do not add multiple IP addresses in the same subnet on different bridge member interfaces. Other interfaces on the bridge should remain with an IP type of None.
It is normally best to avoid such configurations as they can be problematic, but they can also be useful for several types of configurations, such as:
Bridging a wireless interface to a LAN
Transparent firewall (WAN/LAN bridge)
Filtering between portions of a single subnet