This chapter introduced the important concepts of alias interfaces (secondary addresses), VLAN tagging and trunking, and UNIX bridge and STP operation. It also provided introductory information about the UNIX channel bonding (FastEtherChannel) facilities and advanced bridging capabilities of the FreeBSD operating system. Nevertheless, it also concluded that UNIX gateways are not designed for integration into a large switching/bridging hierarchy due to the nature and some limitations of their STP implementation.