Ethernet Addressing
Written on 04:00 by Ahmet Emir
Here’s area we get into how Ethernet acclamation works. It uses the Media Access Control
(MAC) abode austere into anniversary and every Ethernet arrangement interface agenda (NIC). The MAC,
or hardware, abode is a 48-bit (6-byte) abode accounting in a hexadecimal format.
Figure 1.19 shows the 48-bit MAC addresses and how the $.25 are divided.
FIGURE 1 . 1 9 Ethernet acclamation application MAC addresses
The organizationally different identifier (OUI) is assigned by the IEEE to an organization.
It’s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered
address (24 bits, or 3 bytes) that is different (supposedly, again—no guarantees) to anniversary and
every adapter it manufactures. Look carefully at the figure. The high-order bit is the Individual/
Group (I/G) bit. When it has a amount of 0, we can accept that the abode is the MAC address
Organizationally
Unique Identifier (OUI)
(Assigned by IEEE)
24 $.25 24 bits
I/G G/L Vendor assigned
47 46
Ethernet Networking 35
of a accessory and may able-bodied arise in the antecedent allocation of the MAC header. When it is a 1, we
can accept that the abode represents either a advertisement or multicast abode in Ethernet or
a advertisement or anatomic abode in TR and FDDI (who absolutely knows about FDDI?).
The abutting bit is the global/local bit, or aloof G/L bit (also accepted as U/L, area U agency universal).
When set to 0, this bit represents a globally administered abode (as by the IEEE). When the
bit is a 1, it represents a locally absolute and administered abode (as in what DECnet acclimated to do).
The low-order 24 $.25 of an Ethernet abode represent a locally administered or manufacturerassigned
code. This allocation frequently starts with 24 0s for the aboriginal agenda fabricated and continues in
order until there are 24 1s for the aftermost (16,777,216th) agenda made. You’ll acquisition that abounding manufacturers
use these aforementioned six hex digits as the aftermost six characters of their consecutive cardinal on the aforementioned card.
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