1. ARP is a stateless protocol(a stateless protocol is a communications protocol that treats each request
as an independent transaction that is unrelated to any previous request so that the communication consists
of independent pairs of request and response). Network hosts will automatically cache any ARP replies they
receive, regardless of whether or not they requested them. Even ARP entries which have not yet expired will
be overwritten when a new ARP reply packet is received.
2. The communication between 2(or more) hosts in the same LAN did not need the Router to forward the
package.
原文：http://www.cnblogs.com/lxw0109/p/ARP-spoofing.html