Broadcast routing

There are two types of IP broadcast.

One is Limited Broadcast, the other is Directed Broadcast.

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Limited Broadcast

It is what the destination IP address is this case, the destination MAC address of the L2 is ffff: ffff: ffff and all will be F.

Direceted Broadcast

This is the IP address leaving the NW address part of the IP address as it is, and host address part as all 1. For example, Directed broadcast of is

Well, there is something often mistaken, in devices that are routing configuration of, do you think the packet destined to can be routed by default?

The correct answer is “possible”.For that matter, it is also routed To be precise, if the device “other than” having as directly connected route, it can be routed.

Because set in routing table does not necessarily indicate that there is ahead in the segment. In some cases, there may be two of and to its destination.

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Assuming that there were two addresses of /25 as above, must be programmed to the directed broadcast address, but it is not known for the routing device such a thing. is similar, this is the NW address, but it is unknown for the device that such a thing is directly connected.

In other words, routing entry “ NextHop:XXXX” does not mean that “Beyond the NextHop:XXXX, there is NW address of” but means the “there are hosts assigned the IP of ~”.

If any device is directly connected to , then the destination MAC address is set to ffff: ffff: ffff , and sent as directed-broadcast by the device.

ip directed-broadcast

For example, assuming that IP address “” is assigned to FastEthernet 1/1 .

And add the following settings.

(config-if) # ip directed-broadcast

Then, when the directed-broadcast address of has been reached to the device by routing, it will be broadcasted from the interface which above command is set.

This is what often is used in WAKE on LAN.

Security is not the best (which may act as a reflector of DoS attacks), but is a very useful, depending on usage.

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