ip directed-broadcast

hi ive been reading about when to use and i now understand:


ip directed-broadcast
no ip directed-broadcast

question1.  normally when creating rip, eigrp, ospf & bgp i have never used the above as after reading the above url it appears that it is not needed.  so i am wondering is this only used when routing protocols are not ?
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Soulja53 6F 75 6C 6A 61 Commented:
The only time I have used ip directed broadcast is for Wake On Lan implementation. Otherwise, routers will alway contain broadcast traffic from exiting.

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Soulja53 6F 75 6C 6A 61 Commented:
And no, even with no routing protocol it is not used. ip directed broadcast has nothing to do with routing itself.
mikey250Author Commented:
hi soulja,

yes ive read brief comments about (ip directed-broadcast) being used for (wake on lan) implementation.

yes i also realise after reading my previous url it can come under the category of (security) so doing (no ip directed-broadcast) is used although most routers apparently have it disabled automatically - no problem.

for my own clarification - i suppose from my point of view, if i purchased a device that did not by default set (no ip directed-broadcast)...

...and i was not using any routing protocols or any other command to block a ping command from source to destn for example..

...but due to a new implementation i wanted to do a ping test confirming end to end connectivity from source to destination i presume the ping would be successful....?

...and after that test then presumably i could then add (no ip directed-broadcast) on the interfaces required to ensure no hacker could then try and do something negative like bring my network down.

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Soulja53 6F 75 6C 6A 61 Commented:
A ping is not a broadcast. It is unicast.

"No ip directed broadcast" is normal routing behavior. Broadcasts are contained in broadcast domain.

Enabling it is an exception to normal routing behavior.

That said, a device that didn't have directed broadcast as a feature would not forward broadcasts.
mikey250Author Commented:
hi soulja,

question 1.  you say a "ping is not a broadcast", but i assumed that when someone decides to ping an actual broadcast address like below as just one example then that was how ?


i can appreciate what it is but this is where i fall short of putting it into practical terms.!
nociSoftware EngineerCommented:
When addressing the broadcast address you also need to specify -b to provoke a broadcast ping. (YMMV on the OS involved though. Linux + iputils has this behaviour).
Otherwise ping is unicast.
mikey250Author Commented:
hi noci,

question 1.  please explain what:  ping x.x.x.x -b for clarity  ?

question 2.  what is ymmv on the os involved though.  linux + iputils has this behaviour  ?

question 3.  yes i know ping is unicast, just trying to understand how a hacker can prompt a broadcast  ?
ping is not related to the fact you broadcast or not. you an even use a broadcast address as the source of a ping. this produces rather efficient smurf-like attacks.

that fact that you use whatever routing protocol is also unrelated to broadcasts. not needed ? as much with and without ospf for example. you can always send a broadcast explicitely to the broadcast address of a different network. it i just blocked by most routers in their default configurations
mikey250Author Commented:
hi,  im not explaining myself correctly so i apologise.

the reason my question has come about is regarding (no ip directed-broadcast & ip directed-broadcast) which i understand as i have been reading my main question url..!

how does a host pc enable a broadcast from source to destination, although i understand what the router does which is send unicast through all routers and the last router that recognises the broadcast address sends a broadcast, but what is triggered from the host pc point of view, is it something manual ?

or is it when a host is part of a dynamic network and maybe an email is sent to another part of a subnetted network then (ip directed-broadcast) comes into play, is that it  ?
broadcasts are not related with email transfer in any way.

the host pc acts pretty much like any router would do : they follow the rules of IP routing.

check if the destination is directly reachable

all ips of the local subnet including the subnet's broadcast address and are directly reachable

all other ips including a different subnet's broadcast address are not

if the address is on the local lan, use either arp or link-level braodcast to send the packet directly to the required hosts

if the destination address is not on the local network, use the routing table to determine which router to send it to

your host and all the routers along the line follow the exact same rules in this respect. actually all the hosts/routers along the way don't even know the packet is a broadcast except for the last one when you use a lan's broadcast address such as 192.x.y.255

the fact that you use dynamic routing is NOT related with this behaviour in any way


if you wonder about the real-world use, broadcasts can be disabled altogether on most networks.

they are mosty used for MDNS-like services, DHCP, and host discovery in netbios networks but the above usually use the general rather than a network-specific broadcast address.

subnet-specific broadcast are hardly ever used and when they are, they usually could be replaced more elegantly with multicast

allowing packets sent/received to/from broadcast addresses through a router is usually asking for trouble and makes you vulnerable to smurf attacks and variations on the same theme while hardly allowing for anything useful

i'm unsure i'm answering your question but hope that helps
nociSoftware EngineerCommented:
@1: ping x.x.x.x -b means that ping should use a broadcast MAC address together with the network broadcast address....

@2: YYMV = Your Milage May Vary (= there are differences between various implementations)
There are different OS's/toolkit libraries  where you don't need the -b flag. For linux + iptools you need the -b flag...

If you instruct your router to do ip-directed broadcast then it will make a broadcast packet if it looks like one (local net broadcast address, or
So a ping from elsewhere: may become a broadcast if it hits your networks with
Normaly this packet won't be routed.

A Hacker is not bound to ping, there are libraries to craft any packet you like and transmit it. Or you can creat an IP raw-socket  and transmit hand-crafted packets.
mikey250Author Commented:
hi and thanks for that advice.  i understand some of what is stated and some of it is confusing but i will cut & paste this info and read it every so often to get a grip on the understanding.
mikey250Author Commented:
advice appreciated!!
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