An IP block list is a security measure used to prevent unauthorized or malicious access to a network or system. It consists of a list of Internet Protocol (IP) addresses that are identified as suspicious, harmful, or unwanted. When an incoming connection attempt matches an address on the block list, the network automatically denies access, helping to protect against threats such as hacking, spam, or distributed denial-of-service (DDoS) attacks. IP block lists are commonly used in firewalls, email servers, and web applications to enhance cybersecurity by filtering out known sources of malicious activity. Administrators regularly update these lists to adapt to evolving threats.