A perimeter firewall is a cybersecurity application that provides a boundary between private and public networks, acting as an external layer of security to mitigate unwanted traffic and cyberattacks. Once implemented, the perimeter firewall employs one or more inspection methods to control traffic as the data attempts to come in and out of the given network. Then, the network perimeter either permits entry or denies entry based on a predetermined set of rules known as access control lists, or ACLs. A perimeter firewall is the outer line of defense to an enterprise’s network security—it acts as the boundary between the business’ internal data and the rest of the public networks, such as the internet.

Though sometimes used interchangeably, a firewall is simply one component of a perimeter firewall, but perimeter firewalls are typically made up of several components. These can include firewalls, border routers, virtual private networks (VPNs), intrusion detection systems (IDSs), intrusion prevention systems (IPSs), demilitarized zones (DMZs), and more.

• Firewall: responsible for filtering out malicious traffic before it penetrates the network perimeter

• Border Router: a router that exists in both the public and the private network tasked with directing traffic in and out of the business’ network

• VPN: responsible for channeling all data traffic through an encrypted virtual tunnel, disguising the business IP address when accessing the public network

• IDS: monitors and reports threats, suspicious activities, and policy violations with system notifications

• IPS: also responsible for monitoring and reporting malicious activity, but can also automatically defend the network upon discovery

• DMZ: a subnet that enables a business to keep the internal network secure while accessing the external network

Perimeter firewalls essentially act as the business’ gatekeeper, controlling incoming and outgoing data using one or more tactics. These methods of information, or “packet,” analysis include static packet filtering, proxy services, or stateful packet inspection. 

• Static packet filtering: a technique of packet inspection that filters traffic based on the network administrator’s predetermined rules. Static packet filtering is one of the most conventional firewall techniques, as it either accepts or blocks packets based purely on the ACLs, which renders it somewhat limited. 

• Proxy services: acting as a gateway between the network’s end-users and the public network, a proxy-based firewall technique makes individual, intermediary connections between the requester and the resource. Proxy services make it more challenging for cyberattackers to see the network, as direct packet transfers are prevented from either side of the perimeter firewall. 

Stateful packet inspection: one of the more common perimeter firewall tactics, stateful packet inspection records outgoing traffic and only allows the packets to return if they’ve already “passed inspection.” This technique prevents IP spoofing and scanning of the private network from external networks or intruders.

Implementing a perimeter firewall comes with several critical advantages to improve your business’ cybersecurity and increase operational efficiency. A perimeter firewall helps your business to monitor incoming traffic; prevent hackers, virus attacks, and spyware; and promote organizational privacy. A network perimeter is your business’ first line of defense, and preventing cyberattacks from external networks is paramount to your company’s safety, privacy, and success.