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This is because the file already designated a device as 
eth0
 (it's referencing a device 
that's not present in the system), so we would need to correct this file ourselves 
in order to reclaim 
eth0
. We would do that by editing the rules file, deleting the 
line that contains the card that's not on the system, and then changing the device 
designation on the remaining line back to 
eth0
.
The new naming scheme is effective as of 
systemd v197
 and later (in case you didn't 
already know from earlier topics in this book, 
systemd
 is the underlying framework 
utilized in Ubuntu for managing processes and various resources). For the most part, 
the new naming convention references the physical location of the network card on 
your system's bus. Therefore, the name it receives cannot change unless you were 
to actually remove the network card and place it in a different slot on the system's 
board, or change the position of the virtual network device in your hypervisor. If 
your machine does include a 
/etc/udev/rules.d/70-persistent-net-rules
 file, it 
will be honored and the old naming convention will be used instead. This is due to 
the fact that you may have upgraded to a newer version of your distribution (which 
features the new naming scheme, whereas your previous version didn't), so that 
your network devices will retain their names, thereby minimizing disruption when 
moving to a newer release.
As a quick overview of how the network names break down, 
en
 is for Ethernet, and 
wl
 is for wireless. Therefore, we know that the example interface I mentioned earlier 
(
enp0s3
) references a wired card. The 
p
 references which bus is being used, so 
p0
 
refers to the system's first bus (the numbering starts at 0). Next, we have 
s3
, which 
references PCI slot 3. Putting it together, 
enp0s3
 references a wired network interface 
card on the system's first bus, placed in PCI slot 3. The exact details of the new naming 
specification are beyond the scope of this chapter (and could even be a chapter of its 
own!), but hopefully this gives you a general idea of how the new naming convention 
breaks down. There's much more documentation online if you're interested in the 
nitty-gritty details (see the 
Further reading
 section). The important point here is that 
since the new naming scheme is based on where the card is physically located, it's 
much less likely to change abruptly. In fact, it can't change, as long as you don't 
physically switch the positions of your network cards inside the case.

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Chapter 10