Simple Question: Is Broadcasting on a Network bad?

[quantass]

Can you tell me, is "Broadcasting" more intense on a network when used then say if i already knew the ip address of the computer i want to communicate to and just sent a message directly to that computer right away? I am just wondering how Boradcasting works....does it send just one message out and all the computers connected to that network just see that datagram on the network because since it has the special "broadcast" address each computer knows it is a message for them (the NIC will read a message from the network directed to its IP address or the broadcast address)? Or does the switch/router/whatever make duplicates of this datagram and spread it to each computer inside that network (i.e. translates the 255.255.255.255 broadcast address to the approparate destination -> each host computer... so if there are 200 host computers that 1 broadcast message gets converted to 200 datagrams that are sent -- one for each host computer inside that network)?

Thanks for the clarification

[Midknyte]

a broadcast is like you yelling across the room at someone versus using a cellphone (or 2 cups and a piece of string, whatever). everyone hears you, but the message is only meant for one person. imagine a whole room of people yelling at each other (like at the NYSE). after a while, you can't hear the other person. that's why Netbeui is not a preferred protocol, among other reasons.

the switch will send the broadcast message to each computer connected to it. the computers are not "listening" to 255.255.255.255. a switch CAN send a message from one computer directly to another, while a hub will just broadcast to all ports. that's one reason switches are better than hubs.

[quantass]

so you're saying that by broadcasting a message on 255.255.255.255 the switch actually takes that message and individually sends it out to each computer connected to that network? I guess then it wouldnt save any more bandwith to use broadcasting then to just contact each host computer one by one.... I thought by broadcasting the message out only one msg would exist on the network and the other host computers would just hear it for all (no network overload of messages since that 1 msg would be heard by all .. but with a switch it seems that wont be true)

[Midknyte]

broadcast = send this to all ports

it would use MORE bandwidth to broadcast than to do direct contact. I think you are confusing broadcasting a message (like netsend) versus a broadcast protocol (like netbeui).

[quantass]

i didnt realize that....

I figured that having the ability of broadcasting onto a special address (255.255.255.255) that all the host computers that were directly connected onto that network would "see" that 1 single message beng sent out and would just process it....For example, lets say i put together a simple network that has no routers/switches...we just have an ethernet cable and all computers are connected to that ethernet cable. Then when one computer broadcasts a message onto that ethernet cable then all the other computers connected to that ethernet cable would see that same message and since it has a broadcast id (all 255's) then it just processes that message. I guess i am wrong ... woops... I still cant wrap my mind around why those same computers in this topology wouldnt be able to see this simple broadcast and just process it right away...there would be no need to send to each computer individually since the message is already seen in the network traffic. I can understand that when it reaches a router then the router would have to rebroadcast that message to the other connected network (if any) but it would just do the same thng...just put the message out (once) and because other hots were connected to that ethernet cable they would all see it.

Btw, thanks for the help!

[Midknyte]

you can do a "broadcast" of an instruction (SIMD) and that would be efficient for that one command. with netbeui and a hub, ALL messages are broadcasted unlike a switch and tcpip which can be point to point. Broadcast protocols cause network "chatter" or noise which slows down the network.

in tcpip, broadcast messages are sent to the broadcast address (192.168.0.255 is common but it depends on your subnetting)

assuming no further subnetting:
x.x.x.0 = network address
x.x.x.1-254 = client addresses
x.x.x.255 = broadcast address

Is that what you are asking?

[quantass]

Hi again, sorry i forgot to add....

Are you also saying that, if i broadcast out onto 255.255.255.255 then that tool isnt really doing that its really taking your network ID and then traversing from 0-255 and sending a datagram to each host ID in turn since each host computer receives a datagram? I understand there is a different protocol that can be more efificnet but i am more intrigued by the built in ability called "Broadcasting" which is 255.255.255.255 that is talked about...when i read up on it it gives me the impression that the one message is sent out and everyone else hears it....i didnt get the impression that that 1 message is duplicated and sent out to all those hosts (bogging down network)...its as if the message is on the network and the hosts' NIC card decides if it wants to pick out and view that message....

For example, packet sniffers give me the impression taht they just "watch" the traffic on the network -- packets going to/there (ones not really meant for that host computer)...And so broadcasting would be just another "single" message, however, because of its special detinstaion address of all 255s this would tell that host computer (and that packet sniffer) that, that msg is being broadcasted. So, bandwidth shouldn't be any greater because there is 1 message. I can imagine that instead of broadcasting a single message to 100 machines i instead sent a message 1 at a time to each of these 100 machines...woudlnt that not only take longer to reach all the machines (the total accum time to get onto 100 machines) then just the single broadcast and the bandwidth would be more?

Thanks for explaining this to a newbie :)

[quantass]

HI again..just noticed your reply just above my last post....i hope my reply doesnt sound redundant ....

Ok, so hubs and netbeui broadcast to all BUT Switches do send the message to each host computer....Ouch so then ideally only send to machines you know and avoid broadcasting if you can when on a network that uses a switch

[Midknyte]

255.255.255.255 signifies all ports, but in tcpip the broadcast IP of your subnet is x.x.x.255. it could be 192.168.217.255 or 10.10.10.255.

1 datagram is sent to the broadcast address and all tcpip clients would receive it. I think that's what you're getting at. that would be more efficient than single messages, which is what I was referring to with SIMD (single instruction multiple data). like a general telling his whole army to attack rather than going to each soldier individually.

when you asked if broadcasting on a network is bad, I instantly thought of broadcast protocols and routing.