View source for Broadcast address

{{short description|Means of addressing all devices on a network with a single transmission}}
{{refimprove|date=May 2013}}

A '''broadcast address''' is a [[network address]] used to transmit to all devices connected to a multiple-access [[communications network]]. A message sent to a broadcast address may be received by all network-attached hosts.

In contrast, a [[multicast address]] is used to address a specific group of devices, and a [[unicast address]] is used to address a single device.

For [[network layer]] communications, a broadcast address may be a specific [[IP address]]. At the [[data link layer]] on [[Ethernet]] networks, it is a specific [[MAC address]].

== IP networking ==<!--other articles and redirects link here. please preserve this anchor if section title changes-->
In Internet Protocol version 4 ([[IPv4]]) networks, broadcast addresses are special values in the host-identification part of an [[IP address]]. The all-ones value was established as the standard broadcast address for networks that support broadcast.{{Ref RFC|919}} This method of using the all-ones address was first proposed by R. Gurwitz and R. Hinden in 1982.<ref>{{Cite IETF|ien=212|title=IP - Local Area Network Addressing Issues|author1=Robert Gurwitz|author2=Robert Hinden|date=September 1982|url=http://www.rfc-editor.org/ien/ien212.txt}}</ref> The later introduction of [[subnet]]s and [[Classless Inter-Domain Routing]] changed this slightly, so that the all-ones value becomes the '''local broadcast address''' and the all-ones host address of each subnet is that subnet's '''directed broadcast address'''.{{Ref RFC|922}}

The directed broadcast address for any IPv4 host can be obtained by taking the [[bit complement]] (bitwise NOT) of the ''subnet mask'' and then performing a [[bitwise OR]] operation with the host's IP address. A shortcut to this process (for common masks using only 0 and 1 bit placements) is to simply take the host's IP address and set all bits in the ''host identifier'' portion of the address (any bit positions which hold a 0 in the subnet mask) to 1. 

As shown in the example below, in order to calculate the directed broadcast address to transmit a packet to an entire IPv4 subnet using the [[private IP address]] space {{IPaddr|172.16.0.0|12}}, which has the subnet mask {{IPaddr||12|netmask=dotted}}, the broadcast address is calculated as {{IPaddr|172.16.0.0}} bitwise ORed with {{IPaddr|0.15.255.255}} = {{IPaddr|172.31.255.255}}. Directed broadcasts always work within a subnet but, for security reasons, many routers disable the forwarding of these by default.<ref>{{cite web |url=https://docs.ruckuswireless.com/fastiron/08.0.60/fastiron-08060-l3guide/GUID-591F51DA-9AC6-4F57-983E-E77BAA3D4F34.html |title=Enabling forwarding of directed broadcasts |access-date=2023-12-07}}</ref>

{| class="wikitable" style="margin:left;"
|+Broadcast address derivation example
! Network IP address breakdown for {{IPaddr|172.16.0.0|12}} !! Binary form !! Dot-decimal notation
|-
| colspan=3 | In '''bold''' below is shown the '''host part (suffix) of the IP address''', with the network address prefix being the non-bold bits to its left. To obtain the broadcast address, the host bits get set to all 1's, while the network address prefix bits remain intact.
|-
| 1. Network IP address
| <code>10101100.0001'''0000.00000000.00000000'''</code>
| 172.16.0.0 
|-
| 2. Subnet mask (The /12 in the IP address in this case means only the left-most 12 bits are 1s, as shown here. This reserves the left 12 bits for the network address (prefix) and the right <code>32 - 12 = 20</code> bits for the host address (suffix).)
| <code>11111111.1111'''0000.00000000.00000000'''</code>
| 255.240.0.0
|-
| 3. Bit complement (bitwise NOT) of the subnet mask
| <code>00000000.0000'''1111.11111111.11111111'''</code>
| 0.15.255.255
|-
| 4. Broadcast address (bitwise OR of 1. Network IP address and 3. Bit complement of the subnet mask. This makes the broadcast address the ''largest possible IP address (and host address, since the host address portion is all 1s) for any given network address.'')
| <code>10101100.0001'''1111.11111111.11111111'''</code>
| 172.31.255.255
|}

{{anchors|255.255.255.255|0.0.0.0}}A special definition exists for the IP address {{IPaddr|255.255.255.255}}. It is the broadcast address of the ''zero network'' or {{IPaddr|0.0.0.0}}, which in Internet Protocol standards stands for ''this network'', i.e. the local network. Transmission to this address is limited by definition, in that it is never forwarded by the routers connecting the local network to other networks.<ref>{{cite web |author=Ed Harmoush |date=June 9, 2022 |url=https://www.practicalnetworking.net/stand-alone/local-broadcast-vs-directed-broadcast/ |title=Local Broadcast vs Directed Broadcast |access-date=2023-12-07}}</ref><ref>{{cite web |url=https://www.packetcoders.io/local-vs-directed-broadcasts/ |title=Local vs Directed Broadcasts |access-date=2023-12-07}}</ref>

IP broadcasts are used by [[BOOTP]] and [[DHCP]] clients to find and send requests to their respective servers.

Internet Protocol version 6 ([[IPv6]]) does not implement this method of broadcast, and therefore does not define broadcast addresses. Instead, IPv6 uses [[IP multicast|multicast]] addressing to the ''all-hosts'' multicast group. No IPv6 protocols are defined to use the all-hosts address, though; instead, they send and receive on particular link-local multicast addresses. This results in higher efficiency because network hosts can filter traffic based on multicast address and do not need to process all broadcasts or all-hosts multicasts.

==Ethernet==
Broadcast is possible also on the underlying [[data link layer]] in [[Ethernet]] networks. Frames are addressed to reach every computer on a given LAN segment if they are addressed to [[MAC address]] {{MACaddr|FF:FF:FF:FF:FF:FF}}. Ethernet frames that contain IP broadcast packages are usually sent to this address.

Ethernet broadcasts are used, among other purposes, by [[Address Resolution Protocol]] to resolve IP addresses to MAC addresses.

==IPX networking==
[[Internetwork Packet Exchange]] (IPX) allows broadcast. A packet with ''network number'' of <kbd>FFFFFFFF</kbd> is sent to all networks available. When the ''node number'' is specified as <kbd>FFFFFFFFFFFF</kbd>, the packet is intended to be received by all hosts in the network.

==AppleTalk==
The [[AppleTalk]] protocol allows broadcast. When the node ID is specified as <kbd>255</kbd>, a packet is sent to all networks available.<ref>{{citation |title=Introduction to AppleTalk |url=https://developer.apple.com/library/archive/documentation/mac/pdf/Networking/Introduction_to_AppleTalk.pdf |publisher=[[Apple Inc.]] |access-date=2021-10-04}}</ref>

==See also==
* [[Default gateway]]
* [[UDP Helper Address]], a router configuration to forward broadcast network traffic across subnet boundaries

==References==
{{reflist}}

[[Category:Network addressing]]

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