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Ethernet
The Ethernet is a protocol of CSMA/CD (Carrier Sense Multiple Access / Collision Detect) type developed in 1973, to the purpose of possession the transmissions on the cable reliable in the moderate traffic conditions.
From here derives the standard IEEE 802.3 of the 1985 (the last revision in 2002) that it is from the IEEE 802 protocols big family.
This family defines standard for lots of net typologies (like Token Ring, Token Bus, WiFi, etc.), that's why thought to divide the OSI second level in two levels, the superior, LLc (Logical Link Control), is common at all the standards, while the inferior part , the MAC (Medium Access Control), is tied at the physical level.
The LLC level offers different services usually entrusted to the superior levels and not predicted from the old Ethernet; for this reason, the TCP/IP architecture uses the old framing (named DIX) that doesn't use LLC, while others protocols use the "official" standard.

Addresses MAC and IP
In an Ethernet net, when a packet is send, any system connected to the same net receipts it; it is necessary to identify an ambiguous mode the recipient and the sender (for the answer). This happen thanks to a 6 byte MAC address, associated at the global level to the sole NIC (Network Interfare Controller) or rather each device (network card or another) able to transmitter and receipt the dates in a LAN (Local Area Network).
But, as we know, a computer is identified also from an address IP, but in the locals network a system can be individuate only through its MAC address and so it exists a conversion protocol between these two (ARP protocol).
A NIC, to send a packet to a node of the same network, identifies the recipient MAC address and proceeds; instead the packet is intended to a network outside this one (for example Internet), the gateway MAC address will be used (for example an ADSL router) which provides to forward the dates.
While the IP address can be configured according to the requirements, the MAC address is wrote in NIC in the production phase and usually can't be modified.

The MAC address is divided in two parts of three byte each one: the first one is assigned by IEE to each society which makes the request; the second one is used to generate the different addresses for each card produced from the same society.
Like a didactic purpose we can use the assigned addresses to the Microchip (from 00:04:A3:00:00:00 to 00:04:A3:FF:FF:FF), but for commercials purposes, the addresses are bought.

The MAC level (Datalink)
Let's see how is constituted a MAC plot, or rather the "confectioned" packet from the datalink level in the IEEE 802.3 standard.

• Preamble: Formed of one sequence of 1 and 0 for permit to the receipt to synchronize with the transmitter;
• SFD: Start-of-Frame Delimiter, signalizes to the receipt which starts the true and own plot.
• Destination: Contains the recipient MAC address .
• Source: Contains the sender MAC address.
• Length/Type: According to the standard 802.3 this ares can adopt two different significants: if the value is minor or equal to 1500, suggests the dates area length where it supposed to be present a LLC packet which will be precessed by the under par homonym, otherwise indicates the third level protocol contained in the dates area; in this last case the MAC plot is a DIX plot, so doesn't follow the LLC transport, but the dates are passed straight at the third level.
• Data: here are contained the date passed from the superior level; the minimum length is of 46 byte, if this limit isn't respected, in the transmission phase the MAC level adds a padding area to load the remaining space.
• FCS Frame Check Sequence, constituted from 4 byte for the errors control (CRC).