Collision Domain and Broadcast Domains
Collision Domain
The “collision domain” describes a network where packet collisions can occur when two devices on a shared network medium send packets simultaneously. The colliding packets are discarded and must be sent again, which reduces network speed and efficiency.
Collisions usually occur in a hub environment because each port on a hub is in the same collision domain. So, all devices connected to the hub are in the same collision domain, and only one device can transmit at a time. All other devices must listen to the network to avoid collisions. Total network bandwidth is shared among all devices.
In contrast to hubs, each port on a bridge, switch, or router has a different collision domain, which reduces and eliminates the possibility of collisions and enables the devices to use full-duplex communication. The full-duplex communication effectively doubles the speed of data capacity. To understand the collision domains, examine the following figure:
You can see that there are eight collision domains marked in the above topology. The hub is in a single collision domain, or all ports of the hub are in a single collision domain, but each port of the router, bridge, and switch is in a separate collision domain.
Broadcast Domain
All the devices in the broadcast domain can be reached via broadcast at the data link layer. A Broadcast Domain can receive any broadcast packet originating from any device within the network segment. All hub and switch ports belong to the same broadcast domain, but all router ports belong to a different broadcast domain.
All ports of the hub and switch are in the same broadcast domain. Hubs and Switches send broadcasts out to all interfaces except the interface on which they were received. Routers do not transmit broadcasts because when a router receives a broadcast, it does not forward it to other interfaces.
Each router interface belongs to a different broadcast domain, and each broadcast is only propagated within its specific domain. Routers separate the boundaries of the broadcast domains. Now, examine the same figure for the broadcast domain.
In the figure above, you can see four broadcast domains marked. All ports on a hub, bridge, and switch are in the same broadcast domain, and all interfaces of the router are in a different broadcast domain.
Layer 2 devices send broadcasts known as ARP to a known IPv4 address on the local network to discover the MAC address. The host can get IP address configuration from the DHCP server using the Dynamic Host Configuration Protocol (DHCP). A large broadcast domain can connect many hosts. A problem with a large broadcast domain is that it generates excessive broadcasts and negatively affects the network.
A large number of Broadcasts also decreases the bandwidth of the network for normal traffic because the broadcast traffic is forwarded to all the devices in the domain. It also decreases the processing power of computers and network devices. Because the computers and network devices need to process all the broadcast packets received, a part of the CPU power is spent on processing the broadcast packets.
So we need to decrease broadcast. To decrease broadcast, we need to enter a router into the network. The router is an expensive device, So it is not possible to add a router to many networks. Another solution for decreasing is subnetting.
Subnetting is the process which decreases network traffic, improves network performance, and makes management very easy. We can isolate network segments very quickly using subnetting. We can also apply security policies, such as which subnets are allowed or not to communicate together. There are different ways of using subnets. Network administrators can also set services into subnets such as:
- Different floors in a building
- Different Organizations
- A different section of the organizations
- Types of Different Devices, such as servers, printers, and hosts
- Any other division that makes sense for the network.
