❗️Why is Idle Scan (-sI) rarely effective on modern systems?

The Idle Scan (-sI) technique relies on the IP Identification (IP ID) field in the IPv4 header.

✔️Older Operating Systems
On older operating systems such as:
Windows XP
Windows 2000
Some older BSD systems
Certain legacy network devices

the IP ID value was globally incremental. Every outgoing IP packet increased the IP ID by one.
Example:

1000
1001
1002
1003
1004

Nmap used these predictable changes to determine whether the zombie host had communicated with the target.
✅ Modern Operating Systems
On modern operating systems such as:
Windows 10 / 11
Windows Server 2019 / 2022
Most modern Linux distributions
macOS
the IP ID is usually not globally incremental.
Depending on the operating system's TCP/IP stack, the IP ID may be:
Randomized
Per-destination
Per-flow
Generated using separate counters
As a result, the predictable behavior required for an Idle Scan is no longer available.

🟢 Why does this matter?
The success of an Idle Scan depends on finding a zombie host with a predictable IP ID sequence.
Since modern operating systems no longer generate IP IDs in a simple sequential manner, finding a suitable zombie host has become much more difficult.

‼️ For this reason, Idle Scan is primarily considered an educational and historical technique.