A broadcast address is a
network address used to transmit to all devices connected to a multiple-access
communications network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, messag ...
. A message sent to a broadcast address may be received by all network-attached hosts.
In contrast, a
multicast address
A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link l ...
is used to address a specific group of devices, and a
unicast address is used to address a single device.
For
network layer
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.
Functions
The network layer provides the means of transf ...
communications, a broadcast address may be a specific
IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
. At the
data link layer on
Ethernet networks, it is a specific
MAC address
A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking techno ...
.
IP networking
In Internet Protocol version 4 (
IPv4) networks, broadcast addresses are special values in the host-identification part of an
IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
. The all-ones value was established as the standard broadcast address for networks that support broadcast. This method of using the all-ones address was first proposed by R. Gurwitz and R. Hinden in 1982. The later introduction of
subnets and
Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR ) is a method for allocating IP addresses and for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet. Its ...
changed this slightly, so that the all-ones host address of each subnet is that subnet's broadcast address.
The broadcast address for any IPv4 host can be obtained by taking the
bit complement
In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic oper ...
(bitwise NOT) of the ''subnet mask'' and then performing a
bitwise OR
In computer programming, a bitwise operation operates on a bit string, a bit array or a binary numeral (considered as a bit string) at the level of its individual bits. It is a fast and simple action, basic to the higher-level arithmetic oper ...
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 broadcast address to transmit a packet to an entire IPv4 subnet using the
private IP address
In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Both the I ...
space , which has the subnet mask , the broadcast address is calculated as bitwise ORed with = .
A special definition exists for the IP address . It is the broadcast address of the ''zero network'' or , 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.
IP broadcasts are used by
BOOTP
The Bootstrap Protocol (BOOTP) is a computer networking protocol used in
Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951.
While some par ...
and
DHCP
The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a cli ...
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
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
A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking techno ...
. Ethernet frames that contain IP broadcast packages are usually sent to this address.
Ethernet broadcasts are used, among other purposes, by
Address Resolution Protocol
The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function ...
to resolve IP addresses to MAC addresses.
IPX networking
Internetwork Packet Exchange (IPX) allows broadcast. A packet with ''network number'' of
FFFFFFFF is sent to all networks available. When the ''node number'' is specified as
FFFFFFFFFFFF, the packet is intended to be received by all hosts in the network.
AppleTalk
The
AppleTalk
AppleTalk is a discontinued proprietary suite of networking protocols developed by Apple Computer for their Macintosh computers. AppleTalk includes a number of features that allow local area networks to be connected with no prior setup or the ...
protocol allows broadcast. When the node ID is specified as
255, a packet is sent to all networks available.
See also
*
Default gateway
A default gateway is the node in a computer network using the Internet protocol suite that serves as the forwarding host ( router) to other networks when no other route specification matches the destination IP address of a packet.
Role
A gateway ...
*
UDP Helper Address A UDP Helper Address is a special router configuration used to forward broadcast network traffic from a client machine on one subnet to a server in another subnet.
Usage example
The Internet Protocol requires every network interface controlle ...
, a router configuration to forward broadcast network traffic across subnet boundaries
References
{{reflist
Network addressing
de:Broadcast
fr:Broadcast
pl:Broadcast