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Protocole SNAP (Sub-Network Access Protocol)

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Le protocole IP (historiquement lié au champ Ethertype), nécessite dans la version IEEE 802.3, une adresse de protocole SAP spéciale. Le protocole SNAP est prévu à cet effet.

Les deux adresses SAP se voient premièrement attribuer la valeur hexadécimale AA.
Cela ne suffit pas : les protocoles concernés nécessitent encore cinq octets.

Les trois premiers octets indiqueront le numéro de protocole (généralement représentatif d' une firme).
Ce numéro de protocole est appelé OUI (Organizational Unique Identifier) : Identificateur unique de Firme.

Puis, on fait suivre par le champ "Ethertype"

Trame Ethernet (DIX) 
Preamble | sdf | DA | SA | T | Data | FCS |
|
Trame 8023 (IEEE) 
| | | | | Trame LLC | | | | |
Preamble | sdf | DA | SA | L | DSAP | SSAP | Ctl | Data | FCS |
|
Trame 8023 avec SNAP 
Preamble | sdf | DA | SA | L | DSAP | SSAP | Ctl | SNAP | Data | FCS |
|

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Document created the 28/01/2004, last modified the 26/10/2018
Source of the printed document:https://www.gaudry.be/en/reseaux-snap.html

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