Is it true that Ethernet card will discard all traffic not addressed to it?

Question

Whose jog is it to filter out the other packets out - network card or network hubs/switches?

Answer 1

JoelKatz has the right idea.

To understand networking it's very helpful to understand the OSI 7 layer model.  In particular the first 4 layers, physical, logical, network, transport.

An Ethernet LAN works very similar to the old party line telephone system.  You could pick up the phone and listen to other peoples conversations, you could only call when somebody else wasn't using the line and your phone would only ring if the call was destine for your phone.   With the advent of switches, a LAN works very similar to the current phone system.

On an Ethernet LAN there are two types of traffic.  Unicast and Broadcast.  An Ethernet interface will listen for it's own MAC address and the broadcast MAC address (all F).  If a layer 2 packet isn't addressed to it, it won't listen to it.

Hubs a.k.a. repeaters are just amplifiers.  They have no brains, they just boost the signal and forward it out every port.   Hubs work only at layer 1 - physical.

Bridges (first generation switches) build an address table by learning (listening) to all the traffic on the LAN and determine which MAC addresses live where.  Then they only forward traffic destine for a specific MAC address out the port it lives on.  They forward broadcast packets.  Switches are just fancy bridges.  Switches work at layer 2 of the OSI model.

Routers still apply the Ethernet rules, but routers also look at layer 3 information, the IP address.  Routers make intelligent decisions where to forward traffic, albeit with a home router the decision is easy.  A little different with large Internet core routers.  Routers also provide a function known as Address Resolution Protocol, ARP keeps a cross reference between MAC addresses and IP addresses.  That way the router knowns which MAC addresses to use when it receives an incoming IP packet.

Answer 2

An Ethernet card, unless set to 'promiscuous' mode, will ignore all packets that are not addressed to it, are not broadcast packets, and are not addressed to a multicast address on its filter list.

Hubs don't filter packets out, ever. Switches can filter out packets if they're sure a port doesn't need them, but they are not required to filter packets out. Extra packets won't do any harm.

Note that this is applicable to modern network interfaces. Some older network interfaces didn't have sophisticated filtering and often the operating system had to put them in promiscuous mode (where they receive every packet on the wire).

Hubs always echo all packets to all ports. (Unless they support more than one speed, in which case they are really switches.)

Typical switches echo all broadcast and multicast packets to all ports. For unicast packets, they will send them to all ports unless they know which port that device is on. In that case, they will send the packet to only that port.

Some switches support loop detection. In that case, they won't send a packet received on a port to a port that goes to the same place the port they received the packet on goes.

This is a slight simplification. If you need more details, ask a more specific question.