Ethernet Cards require LinuxCNC Uspace and the PREEMPT kernel.

Instructions to install Debian 7 and LinuxCNC Uspace
http://linuxcnc.org/docs/2.7/html/getting-started/getting-linuxcnc.html#_installing_on_debian_wheezy_with_preempt_rt_kernel

Stretch DVD http://linuxcnc.org/dists/stretch/

Optional Instructions to download and install Debian 9 and LinuxCNC Uspace with the
desktop of your choice

https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/
drill down to the latest version of the nonfree amd64 iso-cd netinst.iso

Burn to a CD if you have a PCI Ethernet card remove it, setup with the onboard LAN only
Boot from the CD
Graphical Install, Do Not enter a Root Password! Just hit enter
Debian desktop environment, Mate, SSH server,Print server, standard system utilities

after booting to Debian 9 open a terminal
sudo nano /etc/lightdm/lightdm.conf
to log in without your user name a password uncomment and add your user name
autologin-user=yourusername
autologin-user-timeout=0
CTRL X and yes to save and exit.

Open the Synaptic Package Manager
search for linux-image and install linux-image-4.9.0.6-rt
reboot the pc in a terminal type uname -a it should report back PREEMT RT
Linux d9-uspace 4.9.0-6-rt-amd64 #1 SMP PREEMPT RT Debian 4.9.82-1+deb9u3 (2018-03-02) x86_64 GNU/Linux

in a terminal
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install dirmngr
sudo apt-get install software-properties-common
*** to get the current release of LinuxCNC
sudo apt-key adv --keyserver hkp://keys.gnupg.net --recv-key 3cb9fd148f374fef
sudo add-apt-repository "deb http://linuxcnc.org/ stretch base 2.7-uspace"
*** to get the buildbot current build
sudo apt-key adv --keyserver hkp://keys.gnupg.net --recv-key E0EE663E
sudo add-apt-repository "deb http://buildbot.linuxcnc.org/ stretch 2.7-rtpreempt"
or
sudo add-apt-repository "deb http://buildbot.linuxcnc.org/ stretch master-rtpreempt"
sudo apt-get update
sudo apt-get install linuxcnc-uspace

for programming
sudo apt-get install git-core git-gui make gcc

Configure the network adapter to work with an ethernet card
To find the ethernet adapter name
ip link show or ip a

sudo nano /etc/network/interfaces
auto enp0s25 << change to match your interface name
  iface enp0s25 inet static
    address 10.10.10.1
    netmask 255.255.255.0

shutdown and install a second LAN card if you need to connect to the internet

to add open in terminal to caja
sudo apt-get install caja-open-terminal

to be able to edit the menu add mozo
sudo apt-get install mozo
You will find it in System > Control Center > Main Menu

to install qtpyvcp https://qtpyvcp.kcjengr.com/