help

aimens

hi all ..
i have question when we are sender message broadcast , how the pc know that message for him or not

ITNewbie2

Broadcast messages are typically confined to a Local Area Network, meaning a small segment of possibly a larger network for efficiency and preserving bandwidth. Address Resolution protocol (ARP) is a good example of a broadcast message type.

Example:
When a PC used by John wants to communicate with a PC used by Peter. Through DNS, it determines that Peter's IP address is 192.168.1.3. In order to send Peter a message, John's PC needs to know the MAC address of the PC used by Peter. John's PC looks up its ARP cache to compare the target's IP address with the list of MAC address it has. When a match cannot be found, it sends a ARP request for the MAC address of Peter's PC. This is a broadcast message being sent. Inside the ARP packet contains the target IP address. But since everyone on the local area network receives a copy. Only the target that recognize the IP address as being its own responds to the message.

I hope this helps.

aimens

im understand that , but im talking about message from one to all ( broadcast) when we are send the message the mac will be fffffffff ad ip 255
when message send and pc receives how pc know the message for him or not .

in unicast we put the mac address and ip for recevies pc but in broadcast we don't put that

ITNewbie2

aimens wrote: »

im understand that , but im talking about message from one to all ( broadcast) when we are send the message the mac will be fffffffff ad ip 255
when message send and pc receives how pc know the message for him or not .

in unicast we put the mac address and ip for recevies pc but in broadcast we don't put that

Broadcast by its very definition is a message type that's meant for every node inside a broadcast domain. Its true that ffffff is to send to everyone on the LAN, but in the case of a ARP request, the target's IP address is also contained within the ARP packet.

If its not ARP, but instead, a software update across a VLAN, let's say. Then you would be correct. Everyone on the LAN will receive and process the message with no exception. So depending on what you are trying to achieve. It can affect your understanding on how broadcast works.

ITNewbie2

aimens wrote: »

when message send and pc receives how pc know the message for him or not.

To further explain this particular question. When a PC receives a broadcast message, it automatically assumes the message is for him. Unless the broadcast message solicits for a response from a specific node (such as an APR request). Otherwise the PC simply process the message as if its a unicast. This is why its important to define broadcast domains when setting up a network. Imagine a network with thousands of computers and no broadcast domain defined. When a broadcast message is being sent. Every node on the network process the message. This not only clogs up the network bandwidth, but also waste CPU resources on every PC/Router/Switch on the network.

If anyone else would like to chip in on their explanation please feel free to do so.

FritzG

PC1 sends a broadcast message to the local network looking for PC2
All PC's on the local network receive Broadcast message
PC2 accepts the broadcast message
All other PC's DROP the message because the IP address does not match theirs
PC2 responds to PC1 with a UNICAST message that includes the MAC address for PC2 because the IP address does match
PC1 receives a UNICAST message from PC2 with the MAC address of PC2 and PC1 updates its ARP cache
PC1 sends data directly to PC2 using the <mac address:IP address> in its ARP cache

That is my understanding of how this works. Hopefully that's right. Feel free to correct if its wrong.

aimens

my friend say the pc know this message for him by anding the dest mac address with her mac if result equal mac address for pc he respond the message

Yanio

I think your friend is getting slightly confused between layer 2 and layer 3 addresses. AFAIK no AND-ing is done at layer 2. This process refers to layer 3 IP addresses and subnet mask checking.

As mentioned above, by default a host will assume a broadcast IS meant for it. This will be passed up the stack to upper layers where more checks will be done (does the IP address match, am i running a DHCP server etc etc etc). If the host then decides the message wasn't meant for it, it's dropped.

Upshot being; every host in a broadcast domain will do at least some processing on a broadcast message.