If a switch has three computers connected to it and no VLANs are configured, how many broadcast domains and how many collision domains does the switch create?

Introduction / Context:
Understanding broadcast and collision domains is crucial for designing efficient Ethernet networks. Modern switches help reduce collisions compared to hubs, but they do not automatically create separate broadcast domains unless VLANs are configured. This question explores how a basic switch behaves when three computers are connected and no VLANs are defined, which is a common exam scenario.

Given Data / Assumptions:

A single Ethernet switch has three computers connected, one per port.
No Virtual LANs (VLANs) are configured, so the network is a flat Layer 2 segment.
We assume full duplex operation but focus on conceptual broadcast and collision domains.

Concept / Approach:
A broadcast domain is the set of all devices that will receive a broadcast frame sent at Layer 2. By default, all ports on a switch without VLANs belong to the same broadcast domain, so any broadcast from one host reaches all others. A collision domain is the set of devices that can experience collisions with each other in a shared medium environment. Each switch port creates its own collision domain, because a switch forwards frames intelligently and isolates collisions per port. Therefore, with three computers connected to three switch ports, there is one broadcast domain and three separate collision domains.

Step-by-Step Solution:
Step 1: Consider that a switch forwards broadcasts out all ports except the incoming one, unless VLANs separate them.Step 2: Conclude that all three hosts receive broadcasts from any one host, so there is exactly one broadcast domain.Step 3: Recall that switches separate collision domains per port, unlike hubs that share a single collision domain.Step 4: Since there are three active switch ports, each connected to a computer, there are three collision domains.Step 5: Choose the option that states one broadcast domain and three collision domains.

Verification / Alternative check:
Another way to verify is to compare a hub and a switch. A hub with three computers would have one broadcast domain and one collision domain. Replacing the hub with a switch splits the collision domains, so collisions in one port do not affect the others. However, broadcasts still propagate to all ports, so the broadcast domain remains one. This conceptual comparison confirms the correct counts.

Why Other Options Are Wrong:
Option A is wrong because switches do not create separate broadcast domains per port unless VLANs are configured, so there are not three broadcast domains.
Option B is wrong because it again suggests three broadcast domains, which is incorrect in a flat switch with no VLANs.
Option D is wrong because one collision domain is characteristic of hubs, not modern switches with dedicated switch ports per host.

Common Pitfalls:
Students often confuse broadcast domains and collision domains, sometimes thinking they always match in number. Another mistake is to forget that VLANs are required to create multiple broadcast domains on a single switch. Remember that default switches segment collision domains per port but maintain a single broadcast domain across all ports until you introduce VLANs or routers.

Final Answer:
One broadcast domain and three collision domains