Find the broadcast domain boundaries in your own lab

short · 30 min · Objective 1.6

Task

Prove where one broadcast domain ends and the next begins, by sending a broadcast and watching which hosts receive it. Topology diagrams are drawings; this is the measurement behind them.

Steps

  1. Start a capture on Host B that shows broadcast traffic only: sudo tcpdump -i eth0 -n -e ether broadcast.
  2. Start the same capture on Host C, if you built one.
  3. From Host A, generate a broadcast. The simplest is an ARP for an address nobody has: ping -c 1 192.168.10.99. ARP for an unknown host is a broadcast by definition.
  4. Note which capture saw it. Host C — same subnet, same host-only network — sees it. Host B, on the other side of the router, does not.
  5. Now repeat with a directed ping across the router and watch how differently it behaves: ping -c 1 192.168.20.10 produces no broadcast on Host B's segment at all.

Verify

sudo tcpdump -i eth0 -n -e -c 5 'arp' &
ping -c 1 -W 1 192.168.10.99
ip -br link show

On Host A's own segment the ARP request appears with destination ff:ff:ff:ff:ff:ff. Run the same capture on Host B and it stays silent for the full ping. That silence is the broadcast domain boundary, measured.

Notes

This is the property that makes a router a router. Switches extend a broadcast domain; routers terminate one. Every VLAN argument in domain 2, every broadcast storm in domain 5, and the entire reason subnets exist come back to what you just watched happen.

Write down which VMs saw the broadcast and which did not. That two-line result is your lab's topology diagram, derived rather than drawn.