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New Member

About directed broadcast.

I have added an image of the scenario.

The configurations are as follows:

R0

f0/0: 10.0.0.1/24

R1

f0/0: 10.0.0.2/24

e1/0: 192.168.1.1/26

e1/1: 192.168.1.65/26

e1/2: 192.168.1.129/26

e1/3: 192.168.1.193/26

I was interested about directed broadcast.

From R0:

ping 192.168.1.63 (all is ok with wireshark on e1/0)

ping 192.168.1.127 (all is ok with wireshark on e1/1)

ping 192.168.1.191 (all is ok with wireshark on e1/2)

ping 192.168.1.255 (all is ok with wireshark on e1/3)

I had understood that it was possible to broadcast also all subnets at the same time, but I am not able to find the right broadcast address. I had thought that the right address was 192.168.1.255 because it had all 1 in the subnet portion. But as I have a specific subnet with all 1 in the subnet portion, that broadcast is forwarded only on e1/3. So, how is it possible? Why do some books call 192.168.1.255 all hosts in all subnets? Thanks.

2 REPLIES
Purple

Re: About directed broadcast.

   I don't know of a way to ping all those subnets at the same time.   192.168.1.255 works on the last one because that is where it falls within your network scheme . You would have to ping the individual  broadcast addresses for each subnet.  If your space was say a single  /24 space then    you could ping anyone within the 192.168.1.0/24 .   If I'm wrong someone else can chime in ...

New Member

Re: About directed broadcast.

In RFC 922 "BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS" you can read:

"Broadcast to all hosts on a subnetted IP network (Multi-subnet broadcasts):

A distinguished value for the subnet-number part of the IP address is used to denote "all subnets".

Broadcasts to all hosts of a remote subnetted IP network are done just as directed broadcasts to a single subnet."

I do not know if IOS has implemented this service.

It is strange that the authors say "A distinguished value ... to denote all subnets".

May be that, in a scenario like this, a subnet address is reserved to all subnets, so I can configure only

2**n - 1 subnets on the router, where n is the number of bits reserved for the subnet portion of the address.

Then the router advertises this reserved address and when it receives it, the router knows that it has to

broadcast all subnets except one, as one subnet address is used for the service. I am imagining how it could work.

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