255.255.255.255 VS Subnet Broadcast Address & IP Direct Broadcast

[Iluvnetwork]

I am lost :( I don't get when to use 255.255.255.255 as a broadcast ip address and when to use subnet broadcast ip address. I don't get why RIPv1 uses 255.255.255.255 instead of subnet broadcast ip... If you can explain to me the differences between them, Please help me!  

In addition, if you know about the ip direct broadcast, please let me know. I did google it, but couldn't understand :(

[Flavio Miranda · Accepted solution]

Hi @Iluvnetwork

 For network configuration purpose we need to worry about Directed broadcast address to avoid put then as a host IP address. 

 Let´s say for the classic network  192.168.0.0/24 we have:

192.168.0.1 up to 192.168.1.254 as valid hosts and 192.168.1.255 is the directed broadcast and should not be part of host range.

The broadcast address 255.255.255.255 is called Limited Broadcast commonly used by application like DHCP for example.

The main different between then is that you can route Directed Broadcast if you tell your router to do so.

Limited Broadcast can be used on the local segment only by some applications.

-If I helped you somehow, please, rate it as useful.-

[paul driver]

Apologies Flavio

Didnt noticed you posted.

res

paul

[Iluvnetwork]

One more question! What is the difference between no ip direct broadcast and ip direct broadcast? If I command no ip direct broadcast on the router's serial interface, every incoming packets with the destination of direct broadcast ip gets dropped?

[Flavio Miranda]

'no ip directed broadcast' , which is currently the default policy for Cisco IOS ,means that the device will not permit Directed Broadcast pass through it.

 Let´s say you want to know who is up on a specific segment. Then, you ping let´s say 192.168.0.255 address. If "ip directed broadcast" is configured, the router will send this packet on the segment and all devices under this will respond to the ping. You can see that this is a security problem. 

With 'no ip directed broadcast' the router will not forward this ping and will prevent you from discovering who is up

.

-If I helped you somehow, please, rate it as useful.-

[paul driver · Accepted solution]

Hello

My understanding is:
Lets say I am aware of a network  192.168.1.0 /24 = which  Provides a Broadcast address of 192.168.1.255 - And I ping that address, I can use it as a direct broadcast towards that particular subnet and if ip directed broadcast was enabled for the interface connected to that subnet then broadcast traffic would be flooded within that network

You can also ping 192.168.1.255 if your inside the network ( without directed broadcast) and again you send traffic to all hosts in that subnet.

255.255.255.255  equates the all in this network ( the network your residing in) and would be used by the likes of dhcp hosts requesting address allocation.

res
Paul

[Iluvnetwork]

What might happen if ip directed broadcast is disabled and I ping 192.168.1.255 outside of that subnet?

[paul driver]

Hello

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@Iluvnetwork wrote:
What might happen if ip directed broadcast is disabled and I ping 192.168.1.255 outside of that subnet?

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it would get dropped and safe guard you against DDOS like a smurf attack in which the attacker spoofs the ip address of the victims pc and trys flood it with hosts responses from the broadcast request it initiated

res
Paul