Router addressing principle
路由器是一种三层设备，
是使用IP地址寻址，
实现从源IP到达目标IP地址的端到端的服务，
其工作原理如下：
01
第一步
The First Step
1、路由器接收到数据包，提取目标IP地址及子网掩码计算目标网络地址；
The router receives the data packet, extracts the target IP address and subnet mask, and calculates the target network address;
02
第二步
The Second Step
2、根据目标网络地址查找路由表，如果找到目标网络地址就按照相应的出口发送到下一个路由器；
The routing table is searched according to the destination network address. If the destination network address is found, it is sent to the next router according to the corresponding exit;
03
第三步
The Third Step
3、如果没有找到，就看一下有没有默认路由，如果有就按照默认路由的出口发送给下一个路由器；
If not, check whether there is a default route. If there is, send it to the next router according to the exit of the default route;
04
第四步
The Fourth Step
4、如果没有找到就给源IP发送一个出错ICMP数据包表明没法传递该数据包；
If not, send an error ICMP packet to the source IP, indicating that the packet cannot be delivered;
05
第五步
The Fifth Step
5、如果是直连路由就按照第二层MAC地址发送给目标站点。
If it is a direct connection route, it will be sent to the target site according to the MAC address of the second layer.
