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If you and I went in to
business and we manufactured

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network cards that plugged
into ethernet networks,

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one of our responsibilities
in building and manufacturing

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these cards is to make sure that
each of our network adapters

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had burned into it
or programmed into it

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a 48-bit unique address that
that network adapter could

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use as it communicates locally
on that network segment.

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One of the uses for a layer 2
address on a network adapter

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is that it allows a unique way
of identifying that adapter

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on a network segment.

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And these layer 2
ethernet addresses

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are 48 bits in length.

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But you know, it's
kind of a pain

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to talk to about 48 bits
between friends as 101100.

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It's just too tedious.

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So what we do when we
represent those where humans

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are going to be reading it--

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we're usually using
hexadecimal characters,

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12 hexadecimal characters
to represent those 48 bits.

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Another interesting phenomenon
is that these layer 2 addresses

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have lots of different
names, including

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names like an ethernet
address or a layer 2

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address or a physical address.

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Even though this is a
layer 2 type of function

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because the addresses are
going to be part of the layer 2

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frames, I suppose
because they're

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burned into some of
the network adapters.

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And a network adapter
is a physical thing.

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It got the term
physical address.

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But in any case, all
these terms and names

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are referring to the
layer 2 ethernet address.

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And while we're
doing name calling,

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another name that these are
often referred to as are MAC

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addresses-- capital M,
capital A, capital C--

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where that Mac doesn't
stand for Macintosh.

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It stands for Media
Access Control.

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And it's because we have
these layer 2 addresses that

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intelligent devices like a layer
2 switch can take advantage

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of that information--
for example,

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where this frame is coming
from and where it's going to--

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because the source
and destination layer

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2 addresses are inside
that layer 2 frame.

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So a switch can make
intelligent forwarding decisions

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based on that layer
2 address information

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in each of the frames.

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And oftentimes, when
learning something new--

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like, for example, wow,
there's a layer 2 address

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on my network adapter.

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It's often fun to take
a look at what yours is.

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And so here's a couple
of different ways,

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depending on the
platform that you're on,

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that you can look at
your layer 2 address.

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So let's begin on a Linux box.

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If you happen to
be on a Linux box,

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the command would be IF config.

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It'll list your interfaces.

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And as part of that, it's going
to list the hardware address,

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which would be one more
name that we could refer

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to the layer 2 address as--
so ethernet address, layer 2

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address, physical address, MAC
address, or hardware address.

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They've also put some colons in
to help separate two characters

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at a time.

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On a Windows platform, we'd
go ahead and run the command

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ipconfig/all.

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So if you go to the interface
you're concerned about,

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you can look at the
details for that.

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And then just
below that, here we

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have the physical address, which
is that bad boy right there.

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And once again, is represented
by 12 hexadecimal characters

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representing a 48-bit
unique MAC address.

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And even if we're not on
a wired Ethernet network,

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we still use the
concept of a layer 2

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address when working with Wi-Fi.

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So even on a mobile
device, it's going

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to show us a Wi-Fi address,
which once again is

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12 hexadecimal characters.

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They've thrown in
some colons there just

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to make it easier to read.

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And that represents a
48-bit layer 2 address.

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And if we're on a
network device--

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maybe a router or a switch--

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we'd use the appropriate
command there.

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So the commands may vary a
little bit between Juniper

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and HP and Cisco.

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So here on this
Cisco device, I'm

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going to issue the command
show interfaces fast

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ethernet 1/1, which will show
us a lot of details regarding

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that interface,
fast ethernet 1/1,

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including the layer 2 address.

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So we'll press Enter.

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So here, among a lot
of other details,

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it's showing us the address
is this bad boy right there.

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And so in this output,
Cisco is giving us

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four characters and a period.

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So whether a vendor is
using colons or periods

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to separate the numbers,
they're doing so just

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to make it a little
bit easier to read.

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And then one last thing that
I thought would be fun to do

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is let's take a look at a
packet capture in Wireshark

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to take a look at the
layer 2 information

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and take a look at some
ethernet frames that

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are including both the source
and destination layer 2

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addresses in those frames.

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So we'll just pick
this top one here.

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And here in the Layer 2
Information, if we expand that,

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that's showing us regarding
this ping request--

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I'm cheating a little
bit by just seeing this

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is a ping request right here--

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the layer 2 destination
address in that layer 2

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header, this frame, is
this guy right there.

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And the source
address, the individual

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that's sourcing this
frame on this network,

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has this layer 2 address,
this MAC address right here.

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And one of the big benefits
of including the source

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and destination layer 2
addresses inside of frames

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is that a network device--

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for example, a layer 2 switch--

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can now look at those addresses
and make intelligent forwarding

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decisions on the local
network based on where

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that frame needs to go.

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In this Nugget,
we've taken a look

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at layer 2 Ethernet addresses.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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