3

I was studying for the CCNA exam and I read this: " When a directed broadcast packet reaches a router that is directly connected to the destination network , that packet is broadcast on the destination network "

My question is what if the destination network is more than one hop away from where the directed broadcast was generated? can the directed broadcast reach the destination or it's limited to one hop? if there waws more than one hop and the networks were not directly connected using the same router, will the directed broadcast still reach the destination (that could 2hops+ away) ?

I tried to build a small lab in packet tracer to test it but apparently packet tracer doesn't support the ip directed-broadcasts command

Improve this question
1
  • 4
    The packet on the way to the network is identical to a Unicast packet. The only Router that knows it is a directed broadcast is the final router in the path (which will then broadcast it out to the appropriate network. More details here: practicalnetworking.net/stand-alone/…
    – Eddie
    Commented Nov 23, 2021 at 22:31

2 Answers 2

Reset to default
5

When a host sends a packet that is destined to the broadcast address of a different network than the one to which the host is connected, we call that a directed broadcast, and the source host has no idea that it is a broadcast address because the source host does not know the mask on the destination address. In other words, the sending host only broadcasts on its own network, either with the Limited Broadcast address (255.255.255.255), or with the network broadcast address (the last address in the network to which the source host is connected).

Only the last router in the path from the source to the destination knows if the destination address is the broadcast address of the destination network. Hosts and routers before the router of the destination network cannot know that the destination address is the broadcast address of the destination network because network packets do not carry network masks.

The router for the destination network must be configured to allow directed broadcasts because the default is to deny them (per RFC 2644, Changing the Default for Directed Broadcasts in Routers), but nothing else is even aware that it is a directed broadcast.

Improve this answer
2
  • I appreciate the help Ron! I have one more question! "In other words, the sending host only broadcasts on its own network, either with the Limited Broadcast address (255.255.255.255), or with the network broadcast address (the last address in the network to which the source host is connected)." Does that mean that mean that hosts in a different networks cannot broadcast messages in each other's network? the host has to be a part of the network in order to broadcast?
    – Mouad
    Commented Nov 24, 2021 at 6:30
  • A host has no idea of the network mask of a different network, so it does not know the broadcast address of a different network (the local broadcasts cannot be routed). That means a packet sent to the broadcast address of a different network is treated as a unicast packet until it reaches the router for a different network. It is the final router that determines if the packet will be sent to the destination network. By default, it will not let a packet destined to the broadcast address on the network. It can be configured to do so, but will not by default.
    – Ron Maupin ♦
    Commented Nov 24, 2021 at 15:23
0

this is me 4 mins later!

to anyone who has the same question! I got you covered!

When a device that is not directly connected to the destination network forwards an IP directed broadcast. that packet will travel through all and every router to find its destination! and when it reaches the router that has the entry of the destination network in it's routing table, it will propagate it in the network!

and just like that! or lost boy (directed broadcast) found it's home! no matter how many barriers were facing him (routers)

Improve this answer
3
  • "and when it reaches the router that has the entry of the destination network in it's routing table, it will propagate it in the network!" By default, that is incorrect because router must be explicitly configured to forward a directed broadcast onto a directly attached network. Allowing directed broadcasts is a huge security problem, which is why you must configure a router to allow that. In fact, there are routers that cannot be configured to allow directed broadcasts, although almost all enterprise-grade devices can.
    – Ron Maupin ♦
    Commented Nov 23, 2021 at 23:01
  • @RonMaupin I wouldn't call it a "huge" security problem. Disabling directed broadcast is a poor man's substitute for proper ingress filtering. Of course letting them come from the WAN is a terrible idea, but inside your LAN there's no real difference in the DoS potential between ordinary and directed broadcasts.
    – TooTea
    Commented Nov 24, 2021 at 13:18
  • @TooTea, most attacks and sabotage on business networks originate from within the network. A business needs to treat its network users as potentially hostile users because they are. The statistics for business network attacks is that between 80% and 90% originate inside, but businesses often do not disclose such attacks, and we need to carefully peruse public financial records to find such things,. You cannot get the records for privately-held companies, but researchers think it is worse for those businesses because they do not take the precautions of larger, publicly-held companies.
    – Ron Maupin ♦
    Commented Nov 24, 2021 at 15:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.