Final answer:
A firewall is the network access control that determines which IP packets are allowed entry to a network and which are dropped, functioning as a barrier based on security policies.
Step-by-step explanation:
The network access control that determines which IP packets are allowed entry to a network and which are dropped is a firewall. A firewall acts as a barrier between a trusted network and untrusted networks. It filters the incoming and outgoing network traffic based on an organization's previously established security policies. At its most basic, a firewall can prevent unauthorized access while permitting outward communication.
There are different types of firewalls such as packet-filtering firewalls, stateful inspection firewalls, proxy firewalls, and next-generation firewalls, each providing various levels of security and control. A proxy server, mentioned in the list, acts as an intermediary between a user and the internet but does not primarily determine which packets are allowed or dropped. An Intrusion Detection System (IDS) monitors network traffic for suspicious activity and alerts the system or network administrator. In some cases, it can also respond to detected intrusions. A Virtual Private Network (VPN) extends a private network across a public network, enabling users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network, providing a measure of security and privacy.