IP spoofing is an attack where hackers forge the source IP address in packet headers to disguise their identity or impersonate a trusted system. By using forged IP addresses, attackers can bypass IP-based authentication, launch blind attacks, or make malicious traffic appear to come from a legitimate source. Backdoor attacks involve secret access points, birthday attacks exploit hash collisions, and man-in-the-middle attacks intercept communications (none of which primarily involve forging IP addresses).