IP spoofing

IP spoofing is the creation of IP packets using somebody else's IP address as the source address of an IP packet.

Let's take a look at IP headers in an IP packet:

In the following diagram, the attackers represent someone else's IP address:

Anti-spoofing using access lists

A network operator can implement an anti-spoofing access-list filter to prevent packets with incorrect source IP addresses from entering and leaving the network. With the following command, the access list will prevent incoming packets that have the internal IP address from coming in. 220.x.x.x is a network which I own, so I do not expect to encounter any packets coming from outside my own IP address.

Cisco IOS command:

access...