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Networking Fundamentals · Intermediate

Directed Broadcast

A packet sent to the broadcast address of a specific remote subnet, which a router could forward to that subnet.

Definition

Directed broadcasts allow a remote host to reach all hosts on a distant subnet. Because they were abused in Smurf amplification attacks, routers disable directed broadcast forwarding by default (Cisco's no ip directed-broadcast since IOS 12.0). Some wake-on-LAN designs selectively enable it.

Example

Sending to 192.168.50.255 from another network is a directed broadcast to 192.168.50.0/24.

Exam tip

Directed broadcast forwarding is off by default and is a Smurf attack enabler if turned on.

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