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，形象地说就是把packet给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. 单播寻址中，发送方和接收方之间的关联方式是1:1，每一个目标地址独一无二地标识一个接收方端点。

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. 组播又叫做多播。在组播寻址中，发送方与接收方之间的关联方式是1:N或N:N；在一个单一的传输中，数据报被同时发送给多个收件人。组播不同于广播，目标地址指的是可访问节点的子集，不一定是全部。

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. 在广播寻址中，发送方与接收方之间的关联方式是1:ALL。一个发送方的单个数据报被路由到与广播地址相关联的所有可能的多个端点。网络自动复制数据包给广播域里的所有收件人，该广播域通常是整个子网。

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. 选播也叫任播。在选播寻址中，发送方与接收方之间的关联方式是1:1(of N)。路由算法根据最近的距离从组中选择单个接收者。

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. 地域播指的是信息被交付给网络中某个地理位置里的一组收件人。地域播(geocast)寻址是移动网络路由协议使用的一种特殊形式的组播(multicast)寻址方法。

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