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Understanding Ethernet Frames and MAC Addresses

Learn about Ethernet frames, MAC addresses, and how NICs communicate using frame structure and MAC addressing. Discover the importance of CRC for frame integrity verification. Identify NIC manufacturers by analyzing MAC addresses. Resources: IEEE OUI database.

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Understanding Ethernet Frames and MAC Addresses

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  1. Aula 40 – TEC 11ºF Redes de computadores Prof. António dos Anjos

  2. Ethernet – Frame • Preamble • Serve para sincronizar o NIC destino com o NIC fonte. ( 7 bytes com os bits 10101010) • Start • Indica o início do frame propriamente dito. (1 byte com os bits 10101011) • Endereço MAC destino • Endereço MAC da NIC de destino; • Endereço MAC origem • Endereço MAC da NIC de origem; • Protocolo • Código do protocolo encapsulado pelo frame; (e.g. 0x800 == IP) • Dados • Protocolo da camada de Rede (que está encapsulado no frame); • CRC • Cyclic Redundancy Check – Para a verificação da integridade do frame.

  3. Ethernet – MAC Address • Ao receber um frame, o NIC verifica se o MAC de destino corresponde ao seu. Se não corresponder, este é descartado ou caso contrário é passado à camada de rede; • 6 Bytes que definem um endereço único numa NIC; • Os primeiros 24 bits identificam o fabricante da NIC (OUI – Organization Unique Identifier); • O restantes 24 bits são um número de série único, atribuído pelo fabricante; • O fabricante combina os restantes 24 bits com o OUI para definir um endereço único para o interface Ethernet.

  4. MAC Address – Actividade (10 min) • Identifique os fabricantes dos NICs com os seguintes MAC addresses: • 00:0C:6E:00:2F:57; • 00:09:4B:0A:54:E1; • 00:02:2E:05:55:A3; • 00:02:9C:00:98:23; • 00:04:AC:76:E3:EE; • Recursos: • http://standards.ieee.org/regauth/oui/index.shtml

  5. MAC Address – Activ. (Soluções) • 00:0C:6E:00:2F:57 • 000C6E = ASUSTEK COMPUTER INC.; • 00:09:4B:0A:54:E1 • 00094B = FillFactory NV; • 00:02:2E:05:55:A3 • 00022E = TEAC Corp. R& D; • 00:02:9C:00:98:23 • 00029C = 3COM; • 00:04:AC:76:E3:EE • 0004AC = IBM CORP;

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