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TechKnowSurge Catalog Videos What’s a Collision Domain?
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What’s a Collision Domain?

Collision domains are network segments where data collisions can occur, defined by the devices that connect them. Hubs extend a single collision domain, while bridges, switches, and routers each create separate collision domains per port or segment.

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About this video

A collision domain is a portion of a network where data packets can collide when two devices transmit simultaneously. The boundaries of a collision domain are determined by the type of network device in use. Hubs operate at the physical layer and do not segment traffic, so all devices connected to a hub share a single collision domain. Bridges divide a network into two distinct collision domains, preventing collisions on one segment from affecting the other. Switches significantly reduce collision risk by creating an individual collision domain for each port. Because a switch maintains a MAC address table and forwards traffic only to the intended destination port, devices on separate ports cannot cause collisions with one another. A switch with six connected devices, for example, produces six separate collision domains. Routers also function as collision domain boundaries, further segmenting traffic between networks. In contemporary network design, hubs have been almost entirely replaced by switches, which means most networks are automatically partitioned into a large number of isolated collision domains — one per connected device. Understanding how different hardware defines collision domains is foundational knowledge for network design, troubleshooting, and performance optimization.

What you'll learn

What's covered

Collision Domains

Aligned to

Cisco CCNA
1.13 Describe switching concepts
Cisco CCST Networking
4.5 Explain basic switching concepts
CompTIA Network+
1.2 Compare and contrast networking appliances, applications, and functions
CompTIA A+ Core 1
2.2 Compare and contrast common networking hardware

Key terms

Collision Domain
A network segment where data packets can collide with each other when transmitted simultaneously.
Hub
A basic Layer 1 network device that receives a signal on one port and repeats it out every other port simultaneously, creating a single shared collision domain among all connected devices. Hubs have been replaced by switches in modern networks because they waste bandwidth and cause collisions.
Bridge
A network device that connects two or more network segments and filters traffic based on MAC addresses.
Switch
A network device that connects devices within a LAN and forwards traffic based on MAC addresses.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.

Transcript

This really isn't a piece of equipment, but it's a good time to talk about what a collision domain is.

When we have a bridge sitting right here and then we have two hubs, what it does is it splits our network, or segments our network, into two different collision domains. So now collisions can happen on this side and collisions can happen on this side, but they're not going to happen between each other. So we call them collision domains: collision domain number one over here and collision domain number two over here. So how many collision domains are there here? There are two.

Now I'm going to let you go ahead and calculate this one. What we see here is there are three hubs, and then we have a switch right here. So in this example right here, how many collision domains do we have? If you answered three, then you would be correct. We have one collision domain over here because we have a hub, so collisions can happen on that. We have one collision domain down here and one collision domain right here. So, great job if you got that right.

Let's try it again. How many collision domains do you see here? This is a switch right here. And just to let you know, even though these come together right here and they look like they're bundled, it really is going to two separate ports. So calculate how many collision domains there are and count them out.

The answer here is six. And it's six because this is a switch. It keeps track of a MAC address table of all of these devices and then sends traffic based off of those MAC addresses. A switch doesn't allow collisions between the different ports, and since each one of these devices is plugged into a separate port, that's how many collision domains we have. So here would be one of them. This would be two, three, four, five, and six for the router.

Now this is what our networks look like. We really just use switches at this point in time, so we break apart our network into just many, many, many collision domains depending on how many nodes we have on our network.

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