Plug five computers into one box, and somehow each one talks only to the machine it means to reach, not the whole room. That box is almost always an Ethernet switch, and it does its job so quietly that most users never think about it. As a technician, you have to.
CompTIA A+ Core 1 (220-1201) covers network devices in the Networking domain, and the switch is the device you'll meet most often in wired networks. The objective expects you to know what a switch is, how it differs from a hub and a router, the difference between managed and unmanaged switches, and features like Power over Ethernet and VLANs. This article walks through what a switch actually does, how it decides where to send traffic, and the practical details you check on the job and get tested on.
A switch forwards frames to the right port by MAC address
An Ethernet switch is a Layer 2 device. That "Layer 2" label comes from the OSI model, and it tells you the switch makes its decisions using MAC addresses, not IP addresses. A MAC address is the 48-bit hardware address burned into every network interface, usually written as six pairs of hexadecimal digits like 00:1A:2B:3C:4D:5E.
Here's the core behavior. When a device sends data onto the wire, that data is packaged into an Ethernet frame, and every frame carries a source MAC address and a destination MAC address. The switch reads the destination MAC of each incoming frame and forwards it out only the single port where that destination device lives. Everything else on the switch stays quiet. That targeted forwarding is the whole point of a switch, and it's what separates it from older, dumber devices.
Contrast this with what happens if the switch doesn't know where a destination lives yet. If the destination MAC isn't in its table, the switch floods the frame out every port except the one it came in on. That flooding is normal early behavior, not a fault. The switch is asking, in effect, "who has this address?" and it learns from the reply.
In exam terms, remember the anchor facts: a switch operates at Layer 2 and forwards traffic based on MAC addresses. If a question describes a device that connects multiple wired hosts and sends frames only to the intended recipient, it's describing a switch.
The switch builds a MAC address table by watching traffic
A switch isn't preloaded with a map of your network. It learns. Every time a frame arrives, the switch looks at the source MAC address and records which physical port that frame came in on. It stores that pairing in a MAC address table, sometimes called a CAM table (content-addressable memory).
So the process works in two directions at once. The switch reads source addresses to learn where devices are, and it reads destination addresses to decide where to forward. Over the first few frames of activity, the table fills in, and flooding drops off because the switch now knows which port leads to which device.
Entries don't live forever. Each entry has an aging timer, commonly around five minutes on many switches, though the exact value is vendor-configurable on managed hardware. If a device goes quiet and its timer expires, the switch removes the entry. That keeps the table current when devices move to different ports or get unplugged. Treat the specific timer value as a common default rather than a guaranteed standard.
You don't usually configure any of this on a basic switch. It happens automatically, which is why even a cheap unmanaged switch works the moment you plug it in. But understanding the learning process explains real behavior you'll see: the brief flood of traffic when a network first comes up, or why a switch temporarily sends a frame to the wrong place right after a laptop moves to a new jack.
Managed and unmanaged switches serve very different jobs
This distinction shows up constantly on the exam and in purchasing decisions, so learn it clearly.
An unmanaged switch is plug-and-play. It has no configuration interface, no IP address of its own, and no settings you can change. You connect cables and it forwards frames using the MAC-learning process described above. Unmanaged switches are cheap, silent, and common under desks, in small offices, and behind a home router where you just need more ports.
A managed switch adds a control layer. It has its own management interface, reachable through a web page, a command-line console, or SSH, and it lets an administrator configure and monitor the device. That control unlocks features an unmanaged switch simply doesn't have: VLANs, port security, quality of service (QoS), link aggregation, port mirroring for troubleshooting, and Spanning Tree Protocol to prevent loops.
The practical trade-off is cost and complexity against control. A managed switch costs more and requires someone who knows how to configure it, but it gives you segmentation, security, and visibility that a small business or enterprise needs. An unmanaged switch costs less and needs no expertise, but you get exactly one behavior with no tuning.
There's also a middle category some vendors call "smart" or "web-managed" switches. These offer a limited web interface with a subset of managed features, usually VLANs and basic QoS, at a lower price. For A+ purposes, focus on the clean managed-versus-unmanaged split, but know that smart switches exist in the real world.
| Feature | Unmanaged | Managed |
|---|---|---|
| Configuration | None | Web, CLI, or SSH |
| Own IP address | No | Yes |
| VLAN support | No | Yes |
| Port security | No | Yes |
| Typical use | Home, small office | Business, enterprise |
| Relative cost | Low | Higher |
Port count and speed are the first specs you check
When you spec a switch, two numbers dominate: how many ports it has and how fast each port runs.
Port counts on desktop and rack switches commonly come in 5, 8, 16, 24, and 48 ports. The 24-port and 48-port models are the workhorses of wiring closets, usually built into a 1U rack-mount chassis. Small 5-port and 8-port units are the ones you'll drop under a desk or in a small back office.
Speed is rated by the Ethernet standard each port supports, and switches almost always auto-negotiate down to whatever the connected device can handle. The common tiers are:
| Standard | Common name | Speed |
|---|---|---|
| 100BASE-TX | Fast Ethernet | 100 Mbps |
| 1000BASE-T | Gigabit Ethernet | 1 Gbps |
| 10GBASE-T | 10 Gigabit | 10 Gbps |
Gigabit is the baseline expectation for modern wired networks.