Typically,an IPaddress refers to an individual host on a particular network.IPalso accommodates addresses that refer to a group of hosts on one or more networks.Such addresses are referred to as multicast addresses,and the act of sending a packet from a source to the members of a (71) group is referred to as multicasting.Multicasting done (72) the scope of a single LAN segment is straightforward.IEEE 802 and other LAN protocols include provision for MAC-level multicast addresses.A packet with a multicast address is transmitted on a LAN segment.Those stations that are members of the (73) multicast group recognize the multicast address and (74) thepacket.In this case,only a single copy of the packet is ever transmitted.This technique works because of the (75) nature of a LAN:A transmission from any one station is received by all other stations on the LAN.
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通常，一个 IP 地址指向某网络上的一个主机。IP 同时也具有指向一个或多个网络中的 一组主机的地址形式，这种地址称为多播地址，而将分组从一个源点发送到一个多播组所有 成员的行为称为多播。在单个局域网段范围内的多播操作相当简单。IEEE 802 和其他局域网 协议都包括了对 MAC 层多播地址的支持。当一个具有多播地址的分组在某个局域网段上传 输时，相应多播组的成员都能识别出这个多播地址，并接受该分组。在这种情况下，只需要 传输一个分组副本。这种技术之所以能行之有效，是因为局域网本身具有广播特性：来自任 何一个站点上的传输都会被局域网中的所有其他站点接收到。