In Debian Jessie, a typical `/etc/network/interfaces` contains a line that looks like this:

```
auto eth0
    allow-hotplug eth0
    iface eth0 inet dhcp

```

This causes `eth0` to be configured when `ifup -a` is invoked and allows DHCP to assign an IP address. However, when I open up `/etc/network/interfaces` in Ubuntu 16.04 (Xenial), I observe that no such directives are present:

```
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

```

What happened to `eth0` (or its Ubuntu equivalent `enp4s0`)? I have Xen installed, if that makes any difference.