First, read wiki about iwlwifi.

For Debian 8 Jessie,

1.Add “non-free” component to /etc/apt/sources.list, for example:

# Debian 8 "Jessie"
deb jessie main contrib non-free

2.Update the list of available packages and install the firmware-iwlwifi package:

# apt-get update && apt-get install firmware-iwlwifi

3.As the iwlwifi module is automatically loaded for supported devices, reinsert this module to access installed firmware:

# modprobe -r iwlwifi ; modprobe iwlwifi

For Debian 9 Stretch, there is only one difference.

You need change “jessie” to “stretch”.

But if your host OS is Debian Stable, you may find it is still no working, please refer to email.

Because linux-image and firmware-iwlwifi in Debian 8 Jessie do not support Intel Corporation Wireless 8260, you need to upgrade to Debian Stretch. Debian Stretch is frozen, it is not a long time until it becomes stable.

However, if you don’t want to upgrade to Stretch due to stable or other concern, two methods are provided:

1.add jessie-backports software source to your /etc/apt/source.list

# Debian 8 "Jessie"
deb jessie-backports main contrib non-free

2.install linux-image and firmware-iwlwifi from jessie-backports software source.

sudo apt install -t jessie-backports linux-image-amd64 firmware-iwlwifi

You can get similiar instructions from debian-backports.