between the physical layer of the system and the network. You take your network
interface and plug a cable into it, connecting it on the other end into a jack. That’s all
the physical layer. In our cake, the pistachio sits directly on the plate, with nothing
between.
Our next layer, which has to pass through icing and jam so the operating system can
distinguish between one layer and another, is dulce de leche (think caramel made
from milk). This is our
data layer
. The addressing of this layer is done using the
media access control (MAC) address. This address includes 3 bytes that belong to the
vendor (sometimes referred to as the
organizationally unique identifier
, or OUI). The
other 3 bytes, since the entire MAC address is 6 bytes long, are the unique identifier
for your network interface. The two components together are the MAC address. Any
communication on your local network has to happen at this layer. If I want to talk to
you from my dulce de leche to your dulce de leche (because who else would under‐
stand dulce de leche but another dulce de leche), I would need to use the MAC
address because it’s the only address that your network interface and my network
interface understand. The address is physically wired into the interface itself, which is
why it’s sometimes called the physical address. In
Example 2-1
, you can see a MAC
address in the second column from the output of the program
ifconfig
.
Example 2-1. MAC address
ether 52:54:00:11:73:65 txqueuelen
1000
(
Ethernet
)
The next layer we come across, again crossing through our icing and jam to clearly
distinguish one from the other, is Nilla wafer (vanilla), and our
network layer
. At the
Nilla wafer layer (network), we address using IP addresses. This is also the address
that enables us to pass outside our local network. The MAC address never passes out‐
side the local network. The IP address does, though. Since we can communicate with
different bakeries, all having cakes designed exactly like ours, using IP addresses, this
is the layer that enables routing. It’s the routing address that allows us to get direc‐
tions from one bakery to another by using the IP address.
Example 2-2
shows an IP
address, which is comprised of 4 bytes, sometimes known as
octets
because they are
each 8 bits long. This is a version 4 IP address. Version 6 IP addresses are 16 bytes
(128 bits) long. As with the earlier example, this is from the output of
ifconfig
.
Example 2-2. IP address
inet 192.168.86.35 netmask 255.255.255.0 broadcast 192.168.86.255
The fourth layer in our cake is the teaberry layer (
transport
). Yes, it’s going to be a
strangely flavored cake, but stay with me. Plus, if you don’t know what teaberry is,
you should find it. Teaberry gum is very good. So, the teaberry layer gives us ports.
This is another form of addressing. Think about it this way. Once you get to the bak‐
