Collision domains and broadcast domains: what they are and how to count them

LANs and shared Ethernet

A LAN (local area network) connects devices in one building or site so they can talk directly. Almost every wired LAN today uses Ethernet (IEEE 802.3). The first Ethernet networks put every computer on one shared coaxial cable, and later on a hub, which simply repeats every bit it receives out of all its other ports. Only one device could transmit at a time, much like a group of people sharing one walkie-talkie channel.

To share the medium, early Ethernet used CSMA/CD (carrier sense multiple access with collision detection). A device listens before it sends. If two devices start at the same moment, their signals clash, both detect the collision, wait a random time and try again. The more devices sharing the medium, the more collisions and the slower everything feels.

Collision domains

A collision domain is the part of a network where two transmissions at once would collide. Everything connected to a hub is one collision domain, however many ports the hub has.

A switch (or a bridge before it) changes this. It receives a whole frame, checks the destination MAC address and sends it only where it needs to go. Each switch port is therefore its own collision domain. When the link between the switch and the device runs full-duplex, both ends can send and receive at the same time on separate wire pairs, so collisions cannot happen and CSMA/CD is switched off. Modern wired networks are full-duplex almost everywhere, so collisions now mostly appear as a symptom of a fault, such as a duplex mismatch where one side runs half-duplex and the other full-duplex.

Wi-Fi is different: all devices on one access point share the same radio channel, so they use a related method, CSMA/CA (collision avoidance).

Broadcast domains

A broadcast is a frame sent to every device on the local network, using the destination MAC address ff:ff:ff:ff:ff:ff. Normal protocols need broadcasts: ARP requests and DHCP Discover messages are both broadcasts. A broadcast domain is the set of devices that receive each other's broadcasts.

Hubs and switches forward broadcasts out of every port, so by default a whole switched network is one broadcast domain. A router does not forward broadcasts between its interfaces, so every router interface is the edge of a broadcast domain. A VLAN (virtual LAN) splits one switch into several broadcast domains, each of which needs a router or Layer 3 switch to reach the others; see VLANs and trunks.

Why care? Every device must process every broadcast it receives. A very large broadcast domain wastes bandwidth and CPU, lets any device reach every other device directly, and turns a switching loop into a network-wide broadcast storm. That is why designers keep each VLAN to a sensible size and separate users, servers, phones and guests.

Counting domains: a worked example

Imagine a router with two interfaces. Interface 1 connects to Switch A, which has six PCs. Interface 2 connects to Switch B, which has four PCs on four ports and an old hub on a fifth port with three PCs plugged into it. No VLANs are configured.

There are 2 broadcast domains, one behind each router interface. If Switch A were then split into two VLANs, routed by the router, there would be three broadcast domains while the collision domain count stayed the same.

Tip: a quick rule: count every switch, bridge and router port in use as one collision domain (a hub and everything on it counts as one); count every router interface or VLAN as one broadcast domain.

Real-world example: a slow printer port

At a small accountancy office, staff complained that printing to one networked copier took minutes. The IT support technician checked the switch port and saw a steadily rising count of late collisions and CRC errors on that port only. The copier's network card had been set by hand to 100 Mbit/s full-duplex years earlier, while the new switch port was set to auto-negotiate. Because the copier no longer negotiated, the switch fell back to half-duplex, so the two ends disagreed about whether collisions were possible. The technician set both sides to auto-negotiate, the counters stopped rising and print jobs went through in seconds. The same office later moved its guest Wi-Fi into its own VLAN so that visitors' devices sat in a separate broadcast domain from the copier and the file server.