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mbzadegan
دوستان همانطور که می دانید MAC Address بصورت AA-BB-CC-DD-EE-FF می باشد واین یعنی 48 بیت (12x4=48)
پس چرا در اکثر کتب CCNA برای MAC تنها 40 بیت ذکرکرده اند ( کتاب مرجع کامل CCNA صفحه 38 قسمت آدرس دهی لایه دوم ) ؟؟
منظورتان از اکثر کتب چیست ؟
من در هیچ رفرنسی به یاد ندارم که 40 بیت ذکر شده باشد !!!!
ICND-1 نگارش دوم صفحه 64 مطلبی با عنوان Ethernet Addressing را مطالعه نمایید.
Ethernet LAN addressing identifies either individual devices or groups of devices on a
LAN. Each address is 6 bytes long, is usually written in hexadecimal, and, in Cisco devices,
typically is written with periods separating each set of four hex digits. For example,
0000.0C12.3456 is a valid Ethernet address.
Unicast Ethernet addresses identify a single LAN card. (The term unicast was chosen
mainly for contrast with the terms broadcast, multicast, and group addresses.) Computers
use unicast addresses to identify the sender and receiver of an Ethernet frame. For instance,
imagine that Fred and Barney are on the same Ethernet, and Fred sends Barney a frame.
Fred puts his own Ethernet MAC address in the Ethernet header as the source address and
uses Barney’s Ethernet MAC address as the destination. When Barney receives the frame,
he notices that the destination address is his own address, so he processes the frame. If
Barney receives a frame with some other device’s unicast address in the destination address
field, he simply does not process the frame.
The IEEE defines the format and assignment of LAN addresses. The IEEE requires globally
unique unicast MAC addresses on all LAN interface cards. (IEEE calls them MAC
addresses because the MAC protocols such as IEEE 802.3 define the addressing details.)
To ensure a unique MAC address, the Ethernet card manufacturers encode the MAC
address onto the card, usually in a ROM chip. The first half of the address identifies the
manufacturer of the card. This code, which is assigned to each manufacturer by the IEEE,
is called the organizationally unique identifier (OUI). Each manufacturer assigns a MAC
address with its own OUI as the first half of the address, with the second half of the address
being assigned a number that this manufacturer has never used on another card.
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