Why do unicast packets not intended to me reach my NIC when I'm in promiscous mode?

Question

When I'm connected to a switch, the switch knows which mac address belongs to which port. So even when I'm in promiscous mode, why do I receive unicast packets that are addressed to different mac addresses?

What I should get would be broadcasts and multicasts and unicasts to my mac, I don't understand what this promiscous mode is all about?

Answer 1

Promiscuous mode on a NIC means that it wants to see all network traffic, not just ones intended for it. In "normal" mode, your NIC would see broadcasts, multicasts, and unicasts for it, but nothing else.

I guess the card could tell the switch it's in promiscuous mode, and then the switch would forward all traffic to it. That's all I can think of.

In normal operations, you don't need promiscuous mode. It's just for network monitoring, diagnosing problems, and so on.

Answer 2

Promiscuous mode was more useful in the days of thin-net and Ethernet hubs. You are right that an Ethernet switch learns which MAC addresses are connected to which ports and segregates traffic accordingly.

Some switches allow one port to be used as a monitoring port, all traffic passing throughthe switch is also transmitted to that port.

Answer 3

The switch doesn't send packets on links it knows do not need them. Unless it is a smart switch that is permanently configured to know that a particular MAC address is assigned to a particular port, it will 'leak' some frames.