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Ethernet Frames

Published Updated 2 min read

An Ethernet frame is the unit of data sent over a wired network. Each frame has a fixed structure: a header identifying the sender and receiver, a payload, and a checksum. This guide walks through that structure field by field.

Anatomy of a frame

Fields in transmission order:

  • Preamble + SFD (8 bytes): Marks the start of a frame. Not visible to software.
  • Destination MAC (6 bytes): Address of the receiving machine.
  • Source MAC (6 bytes): Address of the sending machine.
  • EtherType (2 bytes): Identifies the payload protocol. Common values: 0x0800 (IPv4), 0x86DD (IPv6), 0x0806 (ARP).
  • Payload (46–1500 bytes): Carries the frame’s data. Padded to 46 bytes if shorter.
  • FCS (4 bytes): A CRC-32 checksum over the frame. Frames with a mismatched FCS are dropped.

EtherType or length?

The 2 bytes after the MAC addresses depends on its value:

  • 0–1500: length in bytes
  • 1501–1535: reserved
  • 1536+: EtherType

Almost all modern traffic uses an EtherType.

VLAN tags

An optional 4-byte field between the source MAC and EtherType marks the virtual LAN a frame belongs to. Switches use this to separate traffic on shared infrastructure, so tagged frames appear on most enterprise and data centre networks.

Jumbo frames

Some networks support payloads up to around 9000 bytes, well above the standard 1500-byte MTU. The larger size reduces per-byte overhead on high-throughput workloads — common in data centres and storage networks, rare on consumer ones.

Every device along the path must support it, or oversized frames are dropped.

Interactive explorer

Total: 1465 bytes
Header: 14 bytes
Protocol: IPv4
Byte map of the current frame in software order. Software view shows only the bytes the operating system delivers; the preamble and FCS are stripped by the NIC.
DST (6 bytes)DST0SRC (6 bytes)SRC6TYPE (2 bytes)TYPE12PAYLOAD (1451 bytes)PAYLOAD141465
Preamble + SFD8 bytes · 64 bits

7 bytes of clock sync then 1 byte Start Frame Delimiter. Software doesn't see this.

10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011
Destination MAC6 bytes · 48 bits

Who the frame is addressed to

00:1b:63:35:12:7b
Delivery
unicast
Assignment
globally unique
Source MAC6 bytes · 48 bits

Who sent the frame

b8:27:eb:67:7a:3f
Delivery
unicast
Assignment
globally unique
EtherType2 bytes · 16 bits

Protocol type of the payload

0x0800
Protocol
IPv4
Payload1451 bytes · 11608 bits

The packet handed down from the layer above (IP, ARP, …)

65 7f ff e7 8b 5c 94 2b 9d c7 76 7a ac 04 05 5d 85 04 29 c2 d0 ee 0e 70 cb d1 0c e4 13 c4 43 a4 31 a8 ef 34 84 c4 64 0c 85 b0 e5 9e cf 5c e1 9b 53 49 f8 82 5b 5b 2b 4b 12 88 ea 37 b9 7b 50 4c 1b af f0 ac 9e dd 13 4c 0f 3d 74 fe 2e a7 6c ae 50 60 84 1f 68 71 af 76 53 9e 06 5b 26 fd 2f cc 61 4f bb c7 b6 4c 09 08 45 1f 0b c5 ae 9d 64 7e fe d7 67 9b 09 62 3a 88 b1 6b fe 54 f7 89 8b 97 b7 f5 84 1b c9 24 e0 22 9a 05 37 a6 ee 53 32 cf e4 ce 5b 18 ab 7a b0 79 84 70 a8 4e 05 37 4d b2 f4 62 98 4c 04 bd 98 a5 c3 fb 07 8d 31 6b 24 9d 9f a0 de 20 ed 4f dd b6 17 70 e4 b4 dc b3 8d 69 f1 bc d4 85 14 1c 0e a7 b2 e5 30 21 42 9d 5d 02 75 30 4a 86 a9 73 de 6a 4a e8 ca 05 3b df b8 c8 f4 78 fb 80 ec cb 90 43 46 bd 88 03 f8 c2 63 58 4b 4d a1 7e ad 65 f5 4e 7e 7e e0 c6 5f 93 d8 24 0b 8d be 2b 93 0d b6 f5 ad 73 f8 93 79 df 9e 28 2a 4a 9c 11 0d fd bb bc e7 d6 29 7d 50 90 fb 4f 34 3c d6 3f 05 bb 77 8c ee c6 d3 e8 5e 09 a9 0e 75 16 03 24 20 0f 8d c3 db 3c 3b b7 13 9b 73 4a 28 bd a9 06 57 f9 91 be 30 a9 05 92 f3 08 db e0 31 6e 3e f2 77 8f 12 e5 f5 6e e9 c4 ff 6d 2a f8 61 5b 60 20 8b 5e 7a 53 16 15 26 2c a3 55 a0 e8 8b 97 b8 7c cf e1 50 cb 67 4a 5f 66 04 6e 99 5a 40 46 33 74 8b 4b 35 63 87 af 4d 25 74 7e 21 3a 3b ac 0f f1 9a 4a 8f bf 96 6d 1a 4d dc 46 f7 13 68 62 fe 2d d8 a7 26 9b 89 e9 a7 d5 ed 77 ab 05 3e 05 df bc f0 e4 71 51 ff 50 32 80 91 d2 1e 4d 62 25 5a c1 27 e2 d4 d6 75 10 10 0f 8e 80 a9 48 75 15 ad 80 3a 42 e2 fb 06 7d 3e 02 97 e2 9a 0e 0f 71 fd b2 95 8a 2a d6 bb 2d 24 bc 0b 35 08 21 d7 e4 bd 9e c1 70 34 b2 51 c7 ea fc ac 6c 4f 7c de 83 50 d9 a9 50 e3 6e 94 a7 bd ed 13 a8 a6 a4 8b d2 ab 8c f0 60 7e e5 a9 2d d5 36 93 47 3f 95 bf a1 80 96 15 6f ed ee 45 cf 11 6e 60 f0 39 0c 67 c4 83 ce 7f 92 ac ae 54 55 d5 83 01 ee cf 52 3c a9 12 78 df 67 e9 37 81 7d e1 f1 55 5f 5d ae 45 fa 75 2b 92 51 04 b8 98 a5 1f ec 08 d3 02 0d 57 ef e1 7a 0a 5d fd 09 67 6a b7 90 b8 45 4a 48 ce 18 28 13 77 99 9c 00 06 c2 b8 31 ee cc 49 5f a1 ec 75 9b 6d 18 bc b3 cd 56 4b d1 90 d2 8f e5 89 c4 45 8d d7 07 2e 8a 74 08 ec a6 9d 2b ab 77 a5 fd 75 53 1d 04 45 40 36 bf b0 f1 b2 e8 2d 15 21 9c 38 aa 5d 1e 29 1f 76 45 97 31 52 2f 26 43 f4 9d ee cc d6 66 4c 91 28 77 66 91 bd 7e 96 e3 c3 04 cc 18 9f 54 04 ed 62 00 55 6d 56 ae 2c a7 7d de 0c 34 81 64 9b 08 83 e0 53 13 59 bd 1f 7d 52 ac 09 a8 41 7c 4b 1d ab de 63 1e 33 b9 74 61 a2 7a b1 ef a2 7e f8 0d 95 6f 03 9b 45 ad 1e 08 47 dc 72 37 72 e7 82 b3 ea 9e 1e 51 d1 4f c7 2f e4 0b 7b 72 65 d9 8c ea a2 15 79 99 85 be 6a 31 26 59 5f f5 ec 4c ac c8 ec 15 81 4a d0 0d 89 ed 4b 19 a6 38 5e 91 57 1d 10 ab 2a a4 a3 26 eb ea b7 36 7d b3 e6 52 97 b0 c0 39 7e 91 f7 b1 cb 3f 17 c4 9e d8 2e c1 e2 e6 8f 55 d9 05 5f 62 29 bc 3e 0e 79 8a f5 d0 e3 99 8f ea e0 09 f2 64 03 6b b2 63 4d 3c 56 5d cb de d7 c7 39 8f e0 de 95 02 9c 83 4f 1a 1a b5 49 dd ea 1f 40 ea e9 ee 1f 6d 76 fe 1d b9 25 8b c2 53 85 b6 6d 7e c8 ea b8 bf ac 07 33 20 be 8b 12 89 ad ac f6 6a 0c 54 94 ff 00 8c 63 fd fe 09 59 65 61 84 7a f9 fc f3 32 85 5e 70 1c 6c 9f 74 b4 a8 1a b1 54 0b d2 a2 d4 09 42 c9 a1 16 0e dc 48 31 0e e0 d8 32 41 0c ff bc 18 79 61 82 8a f3 b8 ab 75 28 82 1a be 8f c0 de 03 a9 34 68 a8 d0 ef 29 0f 95 ef a3 ab 4e 41 f9 f9 7a dc 2c d4 e8 ce 9a a6 63 22 29 1a 27 2d 96 1c a0 2c 91 e3 9e b5 a7 10 fd 75 4b ef f0 a9 13 9c af e6 3a 45 a5 c0 25 08 8f 48 9b 94 9b c5 58 b5 21 e3 17 98 4b 80 fa 0e 82 71 89 9a c4 ec f4 f8 5f 43 9a ce db f4 ee af 2d df a9 82 bd 7f 3e 42 07 4c 05 fc 46 6d ed c7 bd ba 96 94 7a 86 06 06 82 27 8b 34 eb 42 5a 57 0f 13 e4 27 b8 ac 47 17 d1 75 f5 d8 21 6a ec a9 00 62 c3 3c f9 e2 6c 84 f0 43 96 05 03 43 9c 59 83 e5 8b d1 8c 58 cc 66 16 a8 86 0e 7d 49 14 fa f7 aa 97 78 ed b4 62 54 da c1 c7 2d 17 05 a1 af 1e 7a f3 77 39 21 88 35 07 a0 c7 bf 58 dc 25 18 74 cf fa 1e 51 11 20 19 df 7e 6c 89 82 79 2e 76 7e f4 11 c1 dd 9a 8a f5 f4 a0 ee 14 25 83 18 96 eb 0e bc 93 68 11 60 cd 5f 25 f7 e3 09 1e 22 e4 2c 31 b2 d0 8d 52 72 04 87 d1 3f 1f a5 4e 60 90 98 bb 75 85 73 e4 11 9f 3d 7f c4 03 d5 4a 08 ea ec c7 2a 7c f9 29 45 c3 80 4e e0 9f 10 28 e5 92 af a8 42 0e c3 67 0e 20 62 3d 16 8a d5 d9 0b 0c 5e 1a b9 8e e0 dc 08 ad 31 7b e5 ae 6d de 4b ee 4f 19 8e 44 b8 25 67 22 ec c6 c6 f3 33 7a 5b fa 03 76 6f e5 a9 7b a1 9e f9 0f f0 a2 35 13 a9 2f 16 e1 5c 2a e9 fa 58 ce d4 de b6 a8 e6 1f 04 41 19 45 ae 7f 43 80 b2 1e 72 dd ab 0a f6 c4 10 db 08 b2 7b 56 0e ae 13 45 10 78 b0 c6 e5 97 99 fe 62 c4 0b 97 96 8a ee 71 cb f1 02 96 f7 5f cd aa 00 c8
FCS4 bytes · 32 bits

Frame Check Sequence, a CRC-32 checksum of the frame. Software doesn't see this.

38 7a ec 01

References