Explain "address spoofing."

What will be an ideal response?

Packet filtering firewalls are susceptible to an attack called "address spoofing." Address spoofing occurs when a filter is tricked into believing a packet is coming from an addressed device different from its true originating source. In such a scenario, the originator's true address has been forged or "spoofed." Because proxies are not used, traffic from a client is sent directly to a server, should that client's traffic pass the firewall. This direct communication between a client and server leaves the server exposed and vulnerable to attack.