Por favor, veja Capítulo 2. O que há de novo no Debian 9 das Notas de Lançamento para o Debian 9 (stretch), PC de 64 bits
2.2.9. New method for naming network interfaces
The installer and newly installed systems will use a new standard naming scheme for network interfaces instead of eth0, eth1, etc. The old naming method suffered from enumeration race conditions that made it possible for interface names to change unexpectedly and is incompatible with mounting the root filesystem read-only. The new enumeration method relies on more sources of information, to produce a more repeatable outcome. It uses the firmware/BIOS provided index numbers and then tries PCI card slot numbers, producing names like ens0 or enp1s1 (ethernet) or wlp3s0 (wlan). USB devices, which can be added to the system at any time, will have names based upon their ethernet MAC addresses.
This change does not apply to upgrades of jessie systems; the naming will continue to be enforced by /etc/udev/rules.d/70-persistent-net.rules. For more information, see /usr/share/doc/udev/README.Debian.gz or the upstream documentation.
veja também Como posso mostrar os nomes antigos da eth0 e também renomear as interfaces de rede no debian 9 stretch?
Eu aconselho você a tomar um pouco de tempo para se familiarizar com o Debian 9 antes de atualizar os sistemas Debian 8 para o Debian 9, e / ou seguir documentação / tutoriais recentes, já que as mudanças são significativas.