Prevention inline at boundaries where blocking is acceptable; detection out of band where it is not, or where the traffic cannot easily be routed through a device.

Prevention stops the attack, which is the point. It requires the appliance to be in the traffic path, which makes it a potential point of failure and a source of latency, and means a false positive blocks legitimate work.

Detection sees a copy of the traffic from a tap or mirror port. It cannot break anything and cannot stop anything, and its value depends entirely on somebody acting on what it reports.

The usual practice is to deploy inline but begin in detection mode, tune for a few weeks against real traffic, then enable blocking for the signatures that have proven reliable - often in stages by confidence level.

Out-of-band monitoring remains valuable for internal segments where inserting an inline device would be disruptive, and for gaining visibility of traffic that no boundary device sees.

Because an untuned system produces so many low-value alerts that the genuine ones are never seen - and that failure is far more common than false blocking.

Default signature sets cover every platform and application the vendor knows about. In any given network, most are irrelevant: signatures for software that is not installed, for protocols not in use, for attacks against systems that do not exist. Each still fires on anything resembling its pattern.

The result is thousands of alerts a day, of which almost none matter. Within weeks nobody looks, and the system provides nothing.

Tuning means disabling irrelevant signature groups, suppressing alerts from known-benign sources such as vulnerability scanners and monitoring systems, adjusting thresholds for noisy detections, and reviewing what remains regularly.

Budget the time: several weeks of iterative work at deployment and a recurring review afterwards. An appliance nobody tuned is an expense, and the vendor's default configuration is a starting point rather than a deployment.

Either traffic stops or it passes uninspected, depending on how it is configured - and that choice should be made deliberately.

Fail-open means that on failure the appliance passes traffic through, usually via a hardware bypass that physically connects the two interfaces. The network keeps working, unprotected. That is the right choice where availability outweighs inspection - most internet boundaries for most organisations.

Fail-closed means traffic stops. Nothing passes uninspected, which suits high-security environments and specific regulated segments where uninspected traffic is unacceptable.

Bypass can be internal to the appliance or provided by an external bypass switch, and the external option is more reliable because it does not depend on the failing device behaving correctly.

Whichever is chosen, ensure monitoring alerts on the bypass activating. A fail-open appliance in bypass looks exactly like a working network, and the discovery that it has been passing uninspected traffic for three months is a bad one.

It does not, unless something decrypts first - which is the main practical limit on this class of product.

Most traffic is encrypted, and an inspection engine sees only the outside of it: addresses, ports, certificate details, sizes and timing. Signatures matching exploit content cannot match what they cannot read.

There are three responses. Decrypt at a firewall or dedicated decryption device and feed the plaintext to the inspection engine. Place the appliance where traffic is already decrypted, such as behind a load balancer that terminates TLS in front of servers. Or rely on the metadata, which supports useful detections - reputation, certificate anomalies, beacon timing patterns and connections to known command-and-control infrastructure - without reading content.

The third is more valuable than it sounds and requires no decryption. But content-based detection needs plaintext, and any deployment plan should be explicit about which category of detection it is actually buying.

Where traffic you care about crosses, which is more than just the internet boundary - and placement determines what it can possibly see.

The internet boundary is the standard position and catches inbound exploitation and outbound command-and-control traffic.

In front of the data centre or server segment it sees attacks against servers from inside the network, which the boundary device never sees.

Between user networks and everything else it sees lateral movement - the phase of an incident where detection is most valuable and least common.

At the boundary of operational technology it sees traffic entering an environment that usually has no other protection.

The constraint is throughput and cost, so most organisations start at the internet edge and add internal coverage where the risk justifies it. Out-of-band monitoring from mirror ports is a cheap way to extend visibility internally without inserting devices into paths.

Mostly in capacity and depth rather than in kind - a dedicated appliance inspects more traffic more thoroughly, and is worth it above a certain scale.

Modern firewalls include intrusion prevention using the same kind of signature engine, and for many organisations that is entirely sufficient. It shares the firewall's policy, its logging and its console, which has real operational value.

A dedicated appliance is chosen when throughput demands exceed what the firewall can inspect without becoming a bottleneck, when the deployment is out of band where a firewall cannot go, when inspection is needed at internal boundaries a firewall does not sit on, or when a specialist product's detection quality is materially better.

The honest guidance for most organisations is to enable and tune the firewall's intrusion prevention first. A tuned integrated engine outperforms an untuned dedicated appliance, and the tuning effort is the same either way.

Send them somewhere they are correlated with other sources and read by somebody with the time to act - which usually means a log platform or a managed service, not the appliance's own console.

An alert in isolation is hard to judge. The same alert alongside the firewall log, the authentication log and the endpoint agent's view becomes an assessable event, which is the argument for forwarding everything to a central platform.

Define what happens for each severity: which alerts are acted on immediately, which are reviewed daily, which are only ever looked at during an investigation. Without that, everything is treated the same and therefore nothing is treated as urgent.

Retention matters. Investigations look back weeks or months, and appliance-local storage holds days.

Where there is no capacity to monitor internally, a managed detection service is a legitimate answer and usually a better one than an appliance whose alerts accumulate unread.