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Packet & bit fields templates

Packet — Ethernet II Frame Header

Two MAC addresses and an EtherType, which is the smallest useful demonstration of a field that wraps across word boundaries: a 48-bit address on a 32-bit word does not fit in one row and is drawn split rather than flagged, because wrapping is the layout working and not a defect. It also declares no extent, so the engine says the trailing sixteen bits of the last word are outside the layout rather than unallocated inside it — two states that must not look the same.

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Ethernet II frame header32-bit words · 4 words · 112 bits laid out · 3 fields · ruler shows ABSOLUTE bit indicesMSB-first: bit 0 is the most significant bit of each 32-bit word, drawn leftmost; indices increase to the right.In both orders the most significant bit of a word is the leftmost column — only the numbering flips.Layout checks passed — 3 fields, 4 words, 112 bits: no gaps, no overlaps, nothing past the last word.MSBLSBword 0bits 0–31+0B012345678910111213141516171819202122232425262728293031word 1bits 32–63+4B3233343536373839404142434445464748495051525354555657585960616263word 2bits 64–95+8B6465666768697071727374757677787980818283848586878889909192939495word 3bits 96–127+12B96979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127outside the layoutDestination MAC…bits 0–47 · 48b…Destination MACbits 0–47 · 48bSource MAC…bits 48–95 · 48b…Source MACbits 48–95 · 48bEtherTypebits 96–111 = 0x0800The layout ends at bit 111; the remaining 16 bits of word 3 are outside it, not unallocated. Declare `words 4` to have them checked.No overlaps: each of the 112 checked bits belongs to at most one field.No gaps: bits 0–111 fully covered by 3 fields.Every field ends inside the 4-word layout.

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title "Ethernet II frame header"
width 32
bitorder msb

0-47:   Destination MAC
48-95:  Source MAC
96-111: EtherType = 0x0800