Question on Broadcasts.

Avocado

Hello, I'm new to networking, and I have a question on broadcasts.

Let's say, there are 4 hosts A, B, C, D, connected to a switch, which is connected to a router then to the Internet. If a broadcast is sent from host a, where does it go?

I would think that the broadcasting message would be sent to the switch then down to other 3 hosts: B, C, and D, but some reading materials suggest that it goes to the router. Can anyone help me clear up this idea please?

advanex1

The broadcast would go to every device on the network except for the one that it was sent from.

So a broadcast from A would go to B, C, D, and the router. However, the router would look at the packet and unless it's an ARP and it needs to reply back via Proxy ARP, it would just discard the packet. Routers do not forward broadcasts.

Avocado

1. While Routers do not forward broadcasts, does it forward ARP broadcasts?
If so,
Does the router handle this ARP broadcast differently if:
a. ARP is trying to resolve MAC address of a host within the network?
b. ARP is trying to resolve MAC address of a host outside the network?

advanex1

Routers do not forward any broadcasts (as far as what you need to know for CCNA Knowledge).

A. If the ARP is specifically looking for the MAC address of the router, the router will reply. If it's looking for anyone else on the network, the router will discard the frame.

B. If ARP is trying to resolve the MAC outside of the network, the router then performs Proxy ARP. This means that it will reply back to the source with the router's interface MAC address and from then on the source will then map that destination IP to that router's MAC address. After that, the packet will travel where it needs to go based on IP/MAC mapping from the other routers and their respective networks.

Avocado

Thank you so much for clearing up this.