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Is 192.168.2.255 a directed broadcast?

Posted on 2004-09-07
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Last Modified: 2012-06-21
Noticed this from one of my hosts
192.168.2.6 ------------> 192.168.2.255

Does this hit every node in the 192.168.2.0 network?  I looked at the data link layer of the packet, and it was FF:FF:FF:FF:FF:FF so i'm assuming it does hit every node.



When a directed broadcast DOES occur, what does the data link layer look like?
Thanks
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Question by:dissolved
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Expert Comment

by:vico1
ID: 12002976
Yes It should be your broadcast address assuming that your Subnet mask is 255.255.255.0.
Take a look at this definition for more help: http://www.atis.org/tg2k/_directed_broadcast_address.html
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Author Comment

by:dissolved
ID: 12005408
What I'm confused about, is that usually broadcasts I see are
192.168.2.5 ---------> 255.255.255.255
0A:44:B3:00:00:00: ---> FF:FF:FF:FF:FF:FF

But the one I saw was 192.168.2.5 -----> 192.168.2.255
0A:44:B3:00:00:00: ---> FF:FF:FF:FF:FF:FF


What's the difference between them?
Thanks
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Expert Comment

by:vico1
ID: 12005682
You always going to have 255.255.255.255 /255.255.255.255

What ever is your network you also will have the broadcast for that network.
in your situation, your network is 192.168.2.0 and therefore broadcast will be 192.168.2.255 with Mask 255.255.255.0
If you type the command "route print" from a command prompt you should see those numbers.

The 192.168.2.5-------> 255.255.255.255 should have the gateway 127.0.0.1
that is a loop for Network test.
in addition you should have 192.168.2.5 -----> 192.168.2.255
these are routes that define where you computer can go

What's the difference between them?
they are both Broadcast addresses.

I hope I was clear enough

Regards,

vico1
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Assisted Solution

by:JMellin
JMellin earned 100 total points
ID: 12005739
A directed broadcast is a IP packet from a host on subnet A with a destination to all hosts on another subnet B. If a host on subnet A send to all hosts on the same A subnet thats a local broadcast. Directed mean a subnet broadcast directet somewhere else.
For example ARP uses local broadcast to find layer 2 (MAC) address information for an IP address.

The Datalink layer, Layer 2 (OSI) does not know or care about any of this. If we ignore multicast this is really simple.
Layer 2 frames can either be adressed to the whole world or to one unique address. The whole world on layer 2 is bound to where there is a router stopping all layer 2 frames without any layer 3 headers (IP header) and only forward based on layer 3 packet information.

A router recieving a packet for all hosts on subnet B (B.255) will act depending on configuration. If allowed it will send out a layer 2 broadcast frame FF:FF:FF:FF:FF:FF with a layer 3 IP header B.255.

Modern routers are'nt allowed to do this because of denial of service problems.

OK?
/Johan
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Author Comment

by:dissolved
ID: 12007809
so in short:

- a directed broadcast is always destined for another network.

-In order for a directed broadcast to work, the router must be configured to allow a 192.168.2.255 address (just an example) to pass.

-layer 2 broadcasts never cross networks
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Author Comment

by:dissolved
ID: 12007817
Vico: are you saying 192.168.2.255   255.255.255.0   is the same as 255.255.255.255???
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by:vico1
vico1 earned 100 total points
ID: 12012004
It is not entirely the same thing I will make a short analogy to explain:

Think of broadcast as  a way to find your friend in a room full of people:
Say that you walk into a room with a few hundred people, Unless that you know exactly where your friend is located in the room you cannot find him.
so you would call out loud your friend name, (that is broadcasting in that room) with 192.168.2.255.

Assuming that they are many rooms with few hundred people interconnected with microphones and speakers then would call out for your friend with the microphone so everyone would hear. (That is broadcasting to all rooms) with 255.255.255.255. That is what happend on a network with No DNS or WINS server.

I hope that helped
Vico1.
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Accepted Solution

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jasperomalley earned 300 total points
ID: 12012499
255.255.255.255 is technically called the "all subnets broadcast address", while 192.168.2.255 would be the "local broadcast address for the network 192.168.2.0 with netmask 255.255.255.0." Since most routers prevent broadcasts to 255.255.255.255 from crossing their boundaries, they effectively do the same thing in most cases.
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