A network firewall is a device or function that filters traffic between two networks (typically between the public internet and a local network) according to a defined set of rules. Connections that match permitted rules pass through; everything else is blocked by default.

In short: Industrial cellular routers from Teltonika, Proroute, Robustel, and others include a built-in firewall as a standard feature. The firewall manages WAN-to-LAN access (preventing internet hosts from reaching LAN devices), LAN-to-WAN access (controlling which devices on the LAN can reach the internet, and what they can reach), and zone-based rules for VLAN-segmented networks. Stateful inspection means the firewall tracks the state of each connection, so return traffic for an outbound request is automatically allowed without an explicit return rule.

For IoT deployments, the firewall is the first line of defence on the WAN side. A device with a public IP and no firewall will be probed by automated scanners within minutes of coming online. Even with a private APN (which substantially reduces exposure), the firewall provides defence in depth.

Common firewall configurations for IoT include blocking all inbound WAN connections except for specific management ports (and only from specific source IPs), allowing outbound connections only to known cloud platforms or VPN endpoints, and using port forwarding sparingly and only with strong authentication on the forwarded service. Default credentials on management interfaces are the single biggest cause of compromised IoT devices, and no firewall configuration replaces the need to change them.