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


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