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In this section we revisit hubs and examine their limitations in the context of layers one through three. This discussion will set things up for the higher-level functions that occur at layer 3. Hubs are layer 1 devices that are pretty much obsolete these days with the widespread availability of inexpensive layer 2 switches. The primary limitation of hubs is that they send data out of all ports. This results in multiple collision domains on a single device, which is highly inefficient, not to mention very insecure. Layer 2 switches have the ability to direct traffic based upon layer 2 MAC addresses. This provides immediate relief in terms of data collisions by providing a single collision domain and isolation at the port level. However, broadcast isolation is not possible with layer 2 switches and network congestion is still present. Routers are the solution to this issue and we'll have a look at them in the next section covering Layer 3 of the OSI reference model.