本文介绍了地址解析协议(ARP)及逆向地址解析协议(RARP)的工作原理。ARP用于将IP地址转换为以太网地址，而RARP则实现相反的功能。两者都使用广播发送请求并采用单播进行响应。此外还提到了代理ARP和无偿ARP的应用。
Concepts:
ARP: Address Resolution Protocol [32bitIP addr->48bit ethernet addr]
RARP: inverse of ARP [48bit ethernet addr->32bit IP addr]
Machenism:
ARP:
Application use host name, through DNS interpreted as IP addr. But the physical layer only use ethernet addr to transmit datagram. So host broadcast ARP request to all the host or router on the same network. If any host or router has the same IP addr it asks for. Then it fills out the ethernet address of the ARP request. And then change the order of source IP addr and source ethernet addr with dest IP addr and dest ethernet addr and unicast this ARP response to the host.
RARP:
RARP request need its IP addr instead of ethernet addr. And router no longer response to this request. There are several RARP server who are responsible for respond this. But only the first response the host receive is count.
Both ARP and RARP are using broadcast to send request and unicast to response.
Proxy ARP:
the router response instead of host. it is transparent to external router.
Gratuitou ARP:
1. determine if the same IP addr has already exists
2. update its mapping infomation
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