Hubs, Bridges and Routers

Hubs or Repeaters, are devices which are used to extend a single LAN. It acts like an amplifier. Whatever noise and signals arrives at, it amplifies and forwards to the rest of nodes. It just extends the network.

Bridges, can be used to connect two LANs together to make it a single LAN. The most important point of a Bridge is that it separates collision domain ( in case of Ethernet). Collisions occur when two hosts talk at the same time. But two hosts separated by a Bridge can talk at the same time, as Bridge separates the "electric part".

Routers, connects multiple LANs together. Routers acts as the boundary for broadcast domain. A broadcast should reach all machines in the LAN, but not beyond. Routers thus significantly increases bandwidth.

A Hub or Repeater, is a physical layer device.
A Bridge is a data link layer (L2) device.
A Router is a network layer (L3) device.

Layer n device

When does we call something a layer 3 or layer 2 device ?

A router is a layer 3 device because it takes forwarding and routing decisions depending on the Layer 3 header. Also Layer 2 header addresses gets changed when it passes through a router.

So whether we can call a Switch - who can create VLANs based on IP addresses a Layer 3 device ? May be or may not. Although it looks into the IP headers, the Layer 2 addresses remain unchanged even after it goes through a VLAN switch. So its still a Layer 2 device.

Disclaimer: I am just understanding networking and discussing myself. My interpretations may be wrong.

Feedbacks I received on this from a networking expert:
A device is in l2, if it does l2 fn, i.e., bridging
device is l3, if it routes
if a switch routes packets, it is a router