在計算機網絡之路由選擇中，單播（unicast），組播(multicast)和廣播(broadcast)可謂家喻戶曉，婦孺皆知。但其實播(cast)有5種，列舉如下：

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. 

什麽是播(cast)?

所謂播(cast), 就是把數據包(packet)傳播出去，形象地說就是cast(投擲)出去。

1. 單播(unicast)

Unicast addressing uses a one-to-one association between a sender and destination: each destination address uniquely identifies a single receiver endpoint.

2. 組播(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. 廣播(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. 選播(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.

參考資料：

1. https://en.wikipedia.org/wiki/Routing#Delivery_schemes
2. https://en.wikipedia.org/wiki/Unicast
3. https://erg.abdn.ac.uk/users/gorry/course/intro-pages/uni-b-mcast.html
4. Introduction to IP Multicast - Cisco

圖說單播，組播，廣播，選播和地域播