Spoofing is a method of:

In an IP spoofing attack, the attacker manipulates the source IP address field in the packet header to impersonate a legitimate or authorized address. This allows malicious traffic to bypass IP-based access controls by masquerading as a trusted source, which is why firewalls use anti-spoofing rules to validate that traffic arriving on an interface matches the expected source network for that interface.

Detecting false authentication attempts describes intrusion detection or account lockout mechanisms, not spoofing; spoofing is about forging packet headers, not authentication credentials.

Hiding an illegal IP behind an authorized address via Port Address Translation describes NAT or PAT, which is a legitimate network function, not a spoofing attack.

Hiding a firewall from unauthorized users describes security-through-obscurity or stealth-mode firewall configuration, which is unrelated to the definition of IP spoofing.