The Network+ book introduces network devices early and then talks about them as if the reader has background details on how each device works on a network. I admit that while I knew the terms router, switch and hub, I did not understand their use in a network from a Collision Domain or Broadcast domain perspective. I’m sure I’m not alone in that. So, as part of my studying, I put together a cheat sheet table. First, let’s define what a network segment is.
- A network segment is a piece of a network that is NOT the backbone of the network. It could be your computer connected via Ethernet cable to a switch. It could be the switch connected via cable to the backbone of the network.
- Collision domain is all the devices that can hear each other talk. If you imagine being at a wedding reception in a big ballroom, you can hear all the guests talking. You won’t hear anyone particularly well as their voices collide with one another. The ballroom is one big collision domain.
- Broadcast domain is all the devices that can hear this special frame called a broadcast frame. It is denoted by a destination MAC address of FF:FF:FF:FF:FF:FF. A broadcast frame is meant to be sent to all devices on a network and is typically used by newly joining devices to find resources like a DHCP server.
| Device | What does it do | At what OSI Layer does the device work | Collision Domain | Broadcast Domain | Modifies Contents |
| Hub | Physically connects network segments together in a star topology so that every segment hears everyone elses traffic.. Everyone shares the same bandwidth. | Layer 1 – Physical | Hubs create collision domains and expand existing collision domains. | All devices connected through the same hub are in the same broadcast domain. | No |
| Switch | Logically connects network segments together BUT only sends frames where they need to go. Network segments get the full bandwidth of the link (only hear their own packets) | Layer 2 – Data-Link | Reduces collision domains. Each port on the switch is its own collision domain | All one broadcast domain | No |
| Router | Connects different networks together | Layer 3 – Network | Reduces collision domains. Each port on a router is its own Collision Domain. | Reduces broadcast domains. By default routers do not forward broadcast packets. | Yes – New Ethernet Frame Possibly new IP header |
| Bridge | aka old school switch | Layer 2 – Data-Link | Same as switch | Same as switch | No |
Switches are responsible for getting a received frame to its next destination. It does the by looking at MAC addresses. Switches create an Address Table which maps the switch port to the MAC address(es) available on that switch port. So, when a frame is received on port 1, the switch looks up the destination MAC address to see what port it should go to. If it doesn’t find a matching port, it will send the frame out to all the ports. Switches inspect but do not modify the Ethernet frame.
Routers are Layer 3 devices that are responsible for moving packets across networks. Routers do this by looking at the IP addresses in the packet and determining which of its interfaces is the best match for the destination network and then sending the packet out the best interface. As Routers need to inspect the IP packet, they must strip off the Ethernet frame header (containing the MAC addresses and other information) from the frame it received so they can look inside at the source and destination IP addresses. After taking a look at the IP addresses, the Router creates a new Ethernet frame (using its MAC as the source MAC and a new destination MAC), encapsulates the IP packet into this new Ethernet frame and recreates the Frame Check Sequence (FCS, basically a checksum for error detection).
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