If a hub has three computers connected to it, how many broadcast domains and how many collision domains exist on this network segment?

Introduction / Context:
Understanding broadcast domains and collision domains is fundamental to Ethernet network design and is tested heavily in Cisco certifications. Hubs are legacy devices that operate at Layer 1 of the OSI model and simply repeat electrical signals out all ports. This behavior affects both how broadcasts propagate and how collisions occur on the network segment.

Given Data / Assumptions:

We have a single hub with three computers connected to it by Ethernet cables.
The hub is an unmanaged, Layer 1 device that does not perform any MAC based switching or segmentation.
There are no routers or switches dividing this segment into multiple broadcast or collision domains.

Concept / Approach:
A broadcast domain is the set of devices that receive each other's Layer 2 broadcasts. A collision domain is the set of devices where Ethernet frames can collide with each other because they share the same physical medium. Hubs do not segment collision domains; all ports on a hub belong to the same collision domain. Similarly, because there is no Layer 3 boundary or VLAN separation, the entire hub based segment is a single broadcast domain.

Step-by-Step Solution:
Consider broadcasts first: if any one of the three computers sends a broadcast frame, the hub repeats it to all other ports. All three computers are therefore in the same broadcast domain; there is one broadcast domain on this segment. Now consider collisions: when two computers transmit at the same time, their signals collide on the shared medium, and the hub repeats the collision. Because the hub provides a shared medium, all three computers share the same collision domain. Thus, for three computers connected to a single hub, there is one broadcast domain and one collision domain.

Verification / Alternative check:
Compare this with a switch: a switch creates one collision domain per port, but still a single broadcast domain per VLAN. If these three computers were connected to a switch in the same VLAN, there would be one broadcast domain and three collision domains. The fact that a hub behaves differently (one collision domain across all ports) confirms that the answer is one broadcast domain and one collision domain.

Why Other Options Are Wrong:
Option a and b claim multiple broadcast domains, which would require routers or VLAN boundaries, not present in a simple hub configuration.
Option c suggests one broadcast domain and three collision domains, which describes a switch, not a hub.
Option e introduces two broadcast domains, which again would require routing or VLANs, not a single hub.

Common Pitfalls:
A common mistake is to assume that each port on any device is a separate collision domain. This is true for switches but not for hubs. Another pitfall is to confuse broadcast domains with collision domains: broadcasts are limited by routers and VLAN boundaries, while collisions are limited by physical or shared media connections. Remember that hubs repeat signals at Layer 1 and do not provide any segmentation.

Final Answer:
A hub with three connected computers creates one broadcast domain and one collision domain.