Ethernet Hub – A device for combining multiple Ethernet segments (typically 10BaseT) into a single segment. All traffic appearing on any port of a hub will be echoed to all of the other ports instantaneously.

Ethernet Switch – A device for connecting multiple Ethernet segments, while maintaining them as separate segments. Unlike a hub, a switch will intelligently route packets to the appropriate port (only) based on the MAC-level (OSI Layer 2) address in the packet.

IP – Internet Protocol, the transport protocol used on the Internet and many private networks.

IPSec – IP Security Architecture, a proposed Internet standard for providing security services at the IP layer level. Some early implementations exist, for example in Cisco routers.

Kerberos – An authentication protocol employing "tickets" generated with private key encryption to authorize transactions between a user and a remote server.

L2TP – Layer 2 Tunneling Protocol, an emerging standard for "tunneling" a variety of protocols across an IP connection. It is being forged as a compromise between Cisco’s Layer 2 Forwarding (L2F) and Microsoft’s (et al) PPTP.

POTS – Plain Old Telephone System (see PSTN), a commonly used acronym for dial-up analog telephone lines.

PPTP – Point-to-Point Tunneling Protocol, a standard developed jointly by Microsoft, U.S. Robotics, 3Com, and others, for tunneling PPP packets across IP connections. This protocol, incorporated into Windows NT, Windows 95/98, and other products, is now being merged with Cisco’s Layer 2 Fowarding (L2F) into a new Internet standard called Layer 2 Tunneling Protocol (L2TP).

PSTN – Public Switched Telephone Network, typically refers to common dial-up telecommunications, often extended to include ISDN.

Router – A device for intelligently switching traffic between multiple networks, based on a stored configuration and sophisticated routing software. Routers can switch traffic based on either MAC-level (OSI Layer 2) or Protocol-level (OSI Layer 3) addresses. These capabilities allow routers to perform security functions, as well as adaptive network reconfiguration.

Sniffer – A "sniffer" is a program that is surreptitiously installed on a computer within a local network. The purpose of the program is to put the computer's network interface into "promiscuous mode" (yes, it's really called that), so that it will receive every packet that appears on the network. The sniffer creates a logfile on the computer's disk containing this network data, which may later be retrieved by the perpetrator and analyzed for information such as passwords or proprietary information.

VPN – Virtual Private Network, a term typically used to refer to the creation of a protected network channel over public networking conduits such as the Internet. Technologies used to create VPNs include L2TP and PPTP.