What is the difference between an IDS and an IPS?
Show the answer
Say this first
An IDS watches a copy of the traffic, from a SPAN port or a network TAP, and raises alerts. It is out of band, adds no latency, and cannot stop the packet that triggered the alert. An IPS sits inline, so every packet passes through it and it can drop, reset or rate-limit malicious traffic before it arrives. The price is latency, a new failure point (so you choose fail-open or fail-closed), and the fact that a false positive blocks real users instead of creating a ticket.
Go deeper
Detection methods
- Signature: known patterns of exploits and malware. Precise for known threats, blind to new ones.
- Anomaly or behavior: deviation from a learned baseline. Finds new things, more false positives.
- Protocol analysis: traffic that breaks the protocol's rules.
- Reputation: known bad IPs, domains and certificates.
Deployment practice
- Start an IPS in detect-only mode, tune out the false positives for your environment, then block on high-confidence signatures first.
- Most IPS today lives inside the NGFW. Standalone sensors such as Snort, Suricata (both can run as IDS or IPS) and Zeek (network monitoring) are common for visibility.
- Encrypted traffic hides payloads. Inspect after TLS decryption, or rely on metadata such as TLS fingerprints, certificates and flow behavior.
- Host-based IDS and endpoint detection see what the network cannot, inside encrypted sessions.
They will ask next
Fail-open or fail-closed?
A business decision. Fail-open (hardware bypass) keeps traffic flowing if the IPS dies, choosing availability. Fail-closed stops traffic, choosing security, and fits high-security segments.
Can an IDS block anything?
Only indirectly: it can send TCP resets or ask a firewall to block the source. Both happen after the first packets have already arrived, and resets do nothing for UDP.
Traps
- Confusing false positives (good traffic flagged) with false negatives (attacks missed).