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hackyshank | Cisco CCNA | Module 3 - Layer 2 Data Link

By: hackyshank | Related Course: Cisco CCNA | Published: June 14, 2016 | Modified: June 14, 2016
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Notepaddata link - layer 2

layer 2 controls the communication on the immediate link between 2 devices . the first device and the next immediately connected devices
diagrams = circle means router , thunder bolt mean serial connection.
data at layer 1 is electrical signals
layer 2 data is in a set structure ; PDU = protocol data unit
pdu is called a frame
addressing scheme at layer 2 – source and destination
most common types at layer 2 is MAC address
MAC Add is 48 bits long – written in hex number
example 0002a abcd 1234
each hex number is 4 bits
first 24 bits in a mac Add is the OUI field = organization unique Identifier – cannot be changed – vendor specific
last 24 bits are defined by the manufacture

7 th bit is called UL bit ( universal local bit)
if that bit is turned off -it is universally unique ; in theory there will not be another address like that one.
if that bit is turned on – it means that the address has been assigned locally.


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