Network Connectivity

Hubs/Concentrators/Repeaters
A hub, also called a concentrator or repeater, is found in star and star-wired ring topology networks. It serves as a central meeting place for cables from computers, servers and peripherals. The hub can be a non-intelligent repeater which simply retimes and reamplifies signals. It can offer intelligence, via network management software, to monitor and control network traffic. Operate at the Physical network layer.

Bridges
As the number of nodes and amount of traffic on a network increases significantly, data transfer can become slow and inefficient. Bridges divide these overburdened networks into smaller segments to ensure better traffic control and more efficient use of bandwidth. The segments remain part of a single logical network. Bridges can also connect a high-performance 100Base-T network with a standard 10Base-T network while maintaining services such as email or printing. Operate at the data link layer.

Routers
Routers are similar to bridges in that they link two or more physically separate network segments. The network segments linked by a router, however, remain logically separate and can function as independent networks. Operate at the network layer of the OSI model.

Routers have access to more network level knowledge than is available to bridges. With this knowledge, routers can perform advanced functions such as calculating the shortest, most economical path between source and destination nodes.

Switch

A switch builds on your existing network infrastructure, increasing bandwidth dramatically. Essentially, an Ethernet switch is a multi-port bridge that provides a dedicated 10 Mbps Ethernet connection between ports. With switches, multiple 10 Mbps connections can be established simultaneously, increasing the aggregate bandwidth of the network. Switches also allow for a high bandwidth Fast Ethernet link to servers or the backbone by interconnecting workgroups with high-speed links. Typically, Ethernet switches are more expensive and have a lower latency than repeaters, and are less expensive than routers and bridges.

Gateways

Gateways are highly complex devices used to link two or more networks with different network architectures. For example, a gateway would provide conversion and translation from Ethernet to IBM's SNA architecture when a PC workstation on a LAN needs to access an IBM® mainframe.

Firewalls

A specialed server that protect your local network from users on the Internet

(Firewalls could fall into the Network Services catagory but conceptually it may be better to think of them as a special hardware device.) A dual-homed host is a computer that has two network interfaces, one connected to an internal LAN and the other to the Internet.

List of firewalls from the International Computer Security Association