Repeaters are physical hardware devices that have a primary function to regenerate the electrical signal by:

Reshaping the waveform
Amplifying the waveform
Retiming the signal

Purpose of a Repeater
The purpose of a repeater is to extend the LAN Segment beyond its physical limits as defined by the Physical Layer's Standards (e.g. Ethernet is 500m for 10Base5).
A LAN Segment is a logical path such as the logical bus used by all 802.3 Ethernet types.
A LAN Segment is given an identification number called a Segment Number or Network Number to differentiate it from other segments.
Hubs are also called Multiport Repeaters or Concentrators.
They are physical hardware devices.
Some Hubs are basic hubs with minimum intelligence - no microprocessors.
Intelligent Hubs can perform basic diagnostics and test the nodes to see if they are operating correctly.
If they are not, the Smart Hubs or Intelligent Hubs will remove the node from the network. Some Smart Hubs can be polled and managed remotely.
Bridges are both hardware and software devices.
They can be standalone devices - separate boxes specifically designed for bridging applications, or they can be dedicated PCs with 2 NICs and bridging software.
Most servers software will automatically act as a bridge when a second NIC card is installed.
Routers are hardware and software devices.
They can be cards that plug into a collapsed backbone, stand-alone devices (rack mount or desktop) or software that would run on a file server with 2 NICs.
One definition of a Gateway is the Hardware/Software device that is used to interconnect LANs & WANs with mainframe computers such as DECnet and IBM's SNA.
Often the router that is used to connect a LAN to the Internet will be called a gateway.
It will have added capability to direct and filter higher layer protocols (layer 4 and up) to specific devices such as web servers, ftp servers and e-mail servers.

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