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一、ARP协议的作用
1、ARP协议在以太网中使用,用来将通信的目标地址解析为MAC地址,跨网段通信时解析出网关的MAC地址。
2、如果计算机Aping计算机B的地址,首先计算机A判断目标IP地址和是否自己在一个网段,若在一个网段,数据链路层封装的目标MAC地址就是计算机C的MAC地址。
3、如果计算机Aping计算机C的地址,首先计算机A判断目标IP地址和是否自己在一个网段,若不在一个网段,数据链路层封装的目标MAC地址就是网关的MAC地址。
二、地址解析协议ARP要点
1、ARP请求分组:包含发送方硬件地址/发送方IP地址/目标方硬件地址(未知时填0) /目标方IP地址。
2、ARP请求发送的是广播地址(路由器不转发ARP请求)。
3、ARP响应分组:包含发送方硬件地址/发送方IP地址/目标方硬件地址/目标方IP地址。
4、ARP分组封装在物理网络的帧中传输。
三、地址解析协议ARP的工作过程
当主机A欲向本局域网上的某个主机B发送IP数据报时,就先在其ARP高速缓存中查看有无主机B的IP地址。
1.如有,就可查出其对应的硬件地址,再将此硬件地址写入MAC帧,然后通过局域网将该MAC帧发往此硬件地址。
2.如没有,ARP进程在本局域网上广播发送一个ARP请求分组。收到ARP响应分组后,将得到的IP地址到硬件地址的映射写入ARP高速缓存中。
四、总结
1、ARP用于解决同一个局域网上的主机或路由器的IP地址和硬件地址的映射问题。
2、如果所要找的主机和源主机不在同一个局域网上,那么就要通过ARP找到一个位于本局域网上的某个路由器的硬件地址,然后把分组发送给这个路由器,让这个路由器把分组转发给下一个网络,剩下的工作就由下一个网络来做。