No matter which type it belongs to, the purpose of routing protocol is the same: senddatagram to destination.
不管是哪个类型，每一种路由选择协议的目的都是一样的：把数据报送到目的地。
2
Suppose Host A, whose address is 10.1.23.3 (therefore belonging to Network 10.1.0.0), wants to send a datagram to Host B, whose address is 10.2.156.34 (therefore belonging to Network 10.2.0.0).
假设主机 A 地址为 10.1.23.3（因此属于网络 10.1.0.0），它想将数据报发送到主机 B，其地址为 10.2.156.34（因此属于 10.2.0.0）。
3
The server can then send a datagram to the address and port for the client to receive, as shown in Listing 5.