Hacking Question:

ARP poisoning can be prevented by following methods:
Packet Filtering:
Packet filters are capable for filtering out and blocking packets with conflicting source address information.
Avoid trust relationship:
Organization should develop protocol that rely on trust relationship as little as possible.
Use ARP spoofing detection software:
There are programs that inspects and certifies data before it is transmitted and blocks data that is spoofed.
Use cryptographic network protocols:
By using secure communications protocols like TLS, SSH, HTTP secure prevents ARP spoofing attack by encrypting data prior to transmission and authenticating data when it is received.