How is a mac address format
MAC Addressing
I'm dr. Mike Murphy I have a PhD in
computer science and I teach computer
science and information systems at
Coastal Carolina University in this
lecture I'm going to discuss Mac
addressing and the assignment of MAC
addresses specifically I'll talk about
what MAC addresses are discuss local and
universal administration and illustrate
how to find a chipset manufacturer using
the organizationally unique identifiers
Mac addressing is a concept provided at
the data link layer for a number of
different networking technologies this
is layer 2 what the MAC address provides
is a layer 2 address that is used to
identify a device on a network segment
now in the case of our layer 2 our data
link layer the MAC address is part of
the media access control sub layer so
it's actually part of our Mac sub-layer
and the addresses that we use here are
typically programmed into network
devices at the time those devices are
manufactured however it is possible to
set a MAC address manually depending on
the capabilities of the system and the
device these addresses are 48 bit
addresses so there's 6 bytes long and
they're written in the network order
using hexadecimal notation with each
bite or octet separated by either a
hyphen or a colon so here's two examples
of this address which is 0 0 a0 d5 1 F 2
e 3 2 in this top example it's written
with hyphens and in the bottom example
it's written with colons both ways are
correct both ways are widely used
occasionally I'll even see some devices
that have labels on them with the MAC
address where there aren't any
separators the between the bytes this
can be quite difficult to read but in
general we'll use either hyphens or
colons to separate the bytes
now these addresses can be universally
or locally administered universally
administered addresses are set by the
device manufacturer locally administered
addresses are set by the local system
administrator let's talk about locally
administered addresses first a MAC
address is locally administered if the
twos bit of its first byte is set to one
so if we look at the first bite of a MAC
address if the twos bit if we take this
first number we convert it from
hexadecimal to binary if we see that
there is a 1 in this next to last
position in the twos place then we know
we have a locally administered address
if we're creating locally administered
addresses say we're working with some
network technologies we need our own
addresses this happens quite frequently
when working with virtual machines for
example the easiest method to create a
locally administered address is just
always use a zero - for the first byte
because then that will stand out as
being a locally administered address to
the system administrator and it will
ensure that whatever part you make up
for the rest of the device for the rest
of the address as long as you don't put
two virtual machines with the same MAC
address on the same network segment this
will be unique and it won't conflict
with any real hardware that you have on
that network now if that twos bit is not
a 1 if that twos bit is a zero then what
we have is what's called a universally
administered address and a universally
administered address is set by the
device manufacturer and it always
follows a pattern in which the first
three bytes are the first three octets
of the address are the manufacturers
organizationally unique identifier or Oh
UI the manufacturer then makes up the
last three bytes of the address
typically this is just going to be a
serial number that counts up by one each
time a new device rolls off the assembly
line
now the organizationally unique
identifier portion is assigned to the
manufacturer by the Institute of
Electrical and Electronics Engineers or
I Triple E I Triple E maintains a master
list of öyou eyes in a considerably
large text file seen here in this
example now chipset and device vendors
so the vendor to whom Ando UI is
assigned typically makes the chip that
implements the network device
functionality or chips could be a multi
chip device could be a single chip
device but that device itself that
network card that that router that
switch whatever device it is may be
rebranded by another manufacturer so one
manufacturer may put their label on the
outside of the box but it could be a
different manufacturer inside that
actually made the component and has the
assigned a UI for the device that's
actually fairly typical so if you want
to try to figure out who made a
particular network device figure out who
the chipset vendor was that made the
actual chip that runs the device it's
possible to look that up using the
devices universally administered MAC
address and searching through the I
Triple E master a UI list now you could
look through that list one link at a
time and try to find the o UI for given
manufacturer but there are search tools
that are available
Wireshark maintains this tool for
example where you can simply plug in the
MAC address of a device click find and
see right down here as a result so this
example the zero 0a 0d v 1f - III - is a
chip manufactured by sierra wireless
incorporated based on that o UI
so to summarize the MAC addresses used
as part of the data link layer for a
number of different network technologies
and that address can be universally
administered or locally administered if
the twos bit of the first byte is one
then we have a locally administered
address and address that we set
ourselves otherwise we have a
universally administered address and
that universally administrated
administered address will consist of an
O UI for the first three bytes and a
vendor-supplied portion typically just a
counter for the second part of the
address thank you for watching for more
information including additional
lectures please visit my website at
wwlp.com due to a high volume of email I
am unable to respond to questions that
are not from Coastal Carolina University
students for admissions information
please visit www.hp.com/recycle unported
license
