| 词条 | Broadcast address | ||||||||||||||
| 释义 |
A broadcast address is a network address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. 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. =={{visible anchor|IP networking}}== In Internet Protocol version 4 (IPv4) networks, broadcast addresses are special values in the host-identification part of an IP address.[1] The all-ones value was established as the standard broadcast address for networks that support broadcast.[1] This method of using the all-ones address was first proposed by R. Gurwitz and R. Hinden in 1982.[2] The later introduction of subnets and Classless Inter-Domain Routing changed this slightly, so that the all-ones host address of each subnet is that subnet's broadcast address.[3][4] The broadcast address for an IPv4 host can be obtained by performing a bitwise OR operation between the bit complement of the subnet mask and the host's IP address. In other words, take the host's IP address, and set to '1' any bit positions which hold a '0' in the subnet mask. For example, for broadcasting 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 {{IPaddr|172.16.0.0}} OR {{IPaddr|0.15.255.255}} = {{IPaddr|172.31.255.255}}.
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. 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 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. EthernetBroadcast 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 by Address Resolution Protocol and Neighbor Discovery Protocol to translate IP addresses to MAC addresses. IPX networkingThe Novell IPX protocol allows broadcast as well. When the network address is specified as FFFFFFFF, a packet is sent to all networks available. When the target node address is specified as FFFFFFFFFFFF, the packet is intended to be received by all hosts in the network. AppleTalkThe AppleTalk protocol also allows broadcast. When the node ID is specified as 255, a packet is sent to all networks available. See also
References1. ^1 {{Cite IETF|rfc=919|title=BROADCASTING INTERNET DATAGRAMS|author1=J. Mogul|date=October 1984}} BroadcastBroadcastBroadcast2. ^IEN 212, IP - Local Area Network Addressing Issues, Robert Gurwitz, Robert Hinden, Bolt Beranek and Newman (BBN) (September 1982) 3. ^{{Cite IETF|rfc=922|title=BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS|author1=J. Mogul|date=October 1984}} 4. ^{{IETF RFC|922}}, Broadcasting Internet Datagrams In the Presence of Subnets, J. Mogul (October 1984) 1 : Network addressing |
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