broadcast...unicast..multicast...I m puzzled !!

[sixth_sense]

hi,

I want to know why an unicast is a category of broadcast? Why its not a broadcast frame itself?

As we know there are 4 types of broadcast:

1.layer 2 broadcast

2.layer 3 broadcast

3.unicast

4.multicast

As i know when a pc wants to contact other pc on local lan. it first lookup its own ARP table for hardware address against particular IP address. if its not there..then a broadcast message is generated, all pc pay attention to this frame and requested actual ip address holder pc respond to it.

- its a normal broadcast.

I came to know, a dhcp client pc looking for DHCP server use to send "unicast" frame. my question is: why this frame is "unicast" ? cuz when the client wake up..it also going through same procedure mentioned above. and in DHCP's case, DHCP server will response to client. so .... where the difference?

** I am also not clear about multicast. :(

Thanks,

[david.bradley]

To get an address lease the client sends a broadcast, because it has no knowlege of the DHCP server(s). Of course it does not have an IP addresses but it does have a MAC address. The DCHP recives the lease request broadcast and replies to the mac address that requested the lease.

The client cannot send a unicast frame to the DHCP server because to do this it would have to know the address of the DHCP server. When the client comes to renew it's lease it can then send a unicast packet to the DHCP server because that server gave it the address, plus at the point is already has it's own IP address.

The only difference between broadcast frame and multicast frames is that the multicast frames are recognised by a group of devices, rather that every device. An example of a group using multicast frames would be router routing updates, the routers listen for the traffic on the specific mluticast address.

Dave

[sixth_sense]

Hi David,

Thanks for ur reply. ok, I assume when client comes to DHCP for renewal, It may send unicast. If so, can't we say each point to point conversation within Lan is unicast ?

Is my idea right? when a pc knows destination's pc's IP and MAC address from ARP table and send exclusively to another pc...only then unicast message are thrown?

[david.bradley]

Hi,

Yes that's correct, if the machine talks to a specific device it does it by unicast.

Dave

[sixth_sense]

Thanks for sharing ur knowledge.