在计算机网络之路由选择中，单播（unicast），组播(multicast)和广播(broadcast)可谓家喻户晓，妇孺皆知。但其实播(cast)有5种，列举如下：
Routing schemes differ in how they deliver messages: 1 unicast delivers a message to a single specific node. 2 multicast delivers a message to a group of nodes that have expressed interest in receiving the message. 3 broadcast delivers a message to all nodes in the network. 4 anycast delivers a message to any one out of a group of nodes, typically the one nearest to the source. 5 geocast delivers a message to a geographic area. Unicast is the dominant form of message delivery on the Internet.
什么是播(cast)?
所谓播(cast), 就是把数据包(packet)传播出去，形象地说就是cast(投掷)出去。
1. 单播(unicast)
Unicast addressing uses a one-to-one association between a sender and destination: each destination address uniquely identifies a single receiver endpoint.
2. 组播(multicast)
Multicast addressing uses a one-to-many-of-many or many-to-many-of-many association; datagrams are routed simultaneously in a single transmission to many recipients. It differs from broadcast in that the destination address designates a subset, not necessarily all, of the accessible nodes.
3. 广播(broadcast)
Broadcast uses a one-to-all association; a single datagram from one sender is routed to all of the possibly multiple endpoints associated with the broadcast address. The network automatically replicates datagrams as needed to reach all the recipients within the scope of the broadcast, which is generally an entire network subnet.
4. 选播(anycast)
Anycast addressing is a one-to-one-of-many association where datagrams are routed to any single member of a group of potential receivers that are all identified by the same destination address. The routing algorithm selects the single receiver from the group based on which is the nearest according to some distance measure.
5. 地域播(geocast)
Geocast refers to the delivery of information to a group of destinations in a network identified by their geographical locations. It is a specialized form of multicast addressing used by some routing protocols for mobile ad hoc networks.
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原文：http://www.cnblogs.com/idorax/p/7190154.html