What is a broadcast address?

A broadcast address is a special IPv4 address used to send packets to all devices within one subnet at the same time.

Short wording for an interview:

A broadcast address is a subnet address in which all bits of the host part are set to 1. It is used to deliver a packet to all nodes of the subnet.

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#Why a broadcast address is needed

Broadcast is used for:

• discovering devices on the network (ARP requests)
• automatic configuration (DHCP Discover)
• layer 2 and layer 3 service protocols
• notifications within a LAN

It allows you to reach all hosts with a single packet without needing to know their addresses.

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#How the broadcast address is calculated

Formula:

Broadcast = all bits of the host part = 1

Example:

#Network 192.168.1.0 /24

• Mask: 255.255.255.0
• Host part = last octet

All bits of the host part set to 1 → 255:

Broadcast = 192.168.1.255

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#Example with a non-standard mask:

Network 10.10.5.0 /26

• Mask: 255.255.255.192
• Network range: 10.10.5.0 - 10.10.5.63

Broadcast = 10.10.5.63

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#Another example:

Network 172.16.4.128 /25

• Host part: 7 bits
• Broadcast = 172.16.4.255

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#Signs of a broadcast packet

• Destination MAC address: FF:FF:FF:FF:FF:FF
• Destination IP address: subnet broadcast, for example 192.168.0.255
• Received by all devices in the subnet
• Not routed → routers do not pass broadcast between networks

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#Types of broadcast transmissions

#1. Local broadcast

Address:

255.255.255.255

The packet is delivered to all devices in the current network.

Used by DHCP.

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#2. Directed broadcast

This is the broadcast of a specific subnet, for example:

It is often blocked by default on routers (due to DDoS threats).

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#Examples of protocols that use broadcast

• ARP: who owns this IP?
• DHCP Discover: looks for a DHCP server
• NetBIOS (in older Windows networks)
• Some device discovery services

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#Why broadcast is not routed

Routers cut off broadcast to:

• protect other networks from "storms"
• avoid overloading channels
• limit the broadcast domain

For cross-network discovery, multicast or a universal service discovery is used.

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#Ready answer for an interview

A broadcast address is a special IPv4 address at which a sent packet is received by all devices within one subnet. It is formed by setting all bits of the host part to 1. Broadcast is used in service protocols (ARP, DHCP), but it is not routed between networks. Example: for the 192.168.1.0/24 network, the broadcast address is 192.168.1.255.

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#Cheat sheet (5 seconds)

• Broadcast = all bits of the host part = 1
• Example: /24 → x.x.x.255
• Used by ARP, DHCP
• Does not pass through routers
• Local broadcast: 255.255.255.255