https://www.freebsd.org/doc/en_US.IS...-bridging.html
a bridge can connect different networks between two computers using just one network interface card (nic) in each computer
as for OS view, a bridge sits on a network card and has possibly many IPs, the network card "does not have the IP"
frames are transferred between network cards: IP is an interface protocol (it's not the only one, but popular), TCP is on top of IP is the "software layer" or delivery agent that uses IP's input/output of frames.
SUMMARY: IP address is used in delivering (parts of frames, packets) to software using "an IP" that is merely associated (possibly negotiated with foreign computers, possibly not) with the hardware
If you use an iMac, it uses BSD-like userland (ifconfig is present). If you use Settings (desktop) to add a network, Apple software automatically uses Bridging to do it (noting other OS'es might use TCP/IP and want one IP per NIC or require tunneling if one has only one NIC for many nets: not so with Bridging or Apple Sierra OS).
