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OKay so I found that this frame has size 72 bytes with no explanation and I can't seem to be able to figure out why .
Ethernet payload will be : ip header + arp packet
Arp packet with ip & ethrnet according to wiki has length 28 bytes.
So we have:
28 + 20 (from the header with no options) = 48 bytes (minimum payload in ethernet is 46 bytes so it's enough (?)) 48 + 14 header + 8 (sfd + preamble) = 70 bytes

What am I missing?

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An ARP packet for 48-bit MAC (like over Ethernet) is 28 bytes, see RFC 826. ARP does not use an IPv4 header but uses its own EtherType of 0x0806.

ARP's size is less than the minimum payload for Ethernet of 46 bytes, so the frame is padded to the minimum frame size of 64 bytes (including 18 bytes overhead[*1]). Note that when you're capturing outgoing frames these may show up without padding, not resembling what actually goes on the wire. Also, many capturing tools omit the FCS.

Preamble, SFD and IPG (7+1+12=20 bytes total) are not part of a frame (L2) but physical-layer (L1) overhead.

[*1] destination MAC (6), source MAC (6), EtherType (2), FCS (4), see IEEE 802.3 Clause 3; the 802.1q option adds 4 bytes, QinQ 8 bytes.

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  • I includeed spf and preable just because they were where mentioned in the answer, in fact the answer was a plain : "72 bytes (including padding, sfd, preamble) " but I could not figure it out. I still need some help : so you are saying that there is no ip header? which leaves us with 28 bytes (so we need padding up to 46 ) 14 + 46 = 60 < 64 so we need more padding --> (we need 4 more bytes exactly) so payload 46 + 4 = 50 , so shouldn't it be 64? What are those 18 you are mentioning? Jan 29 at 16:19
  • If that's a homework question, those are explicitly off-topic here, see the help center. You've asked about a frame and a frame doesn't include anything from the physical layer. All your questions are in the answer.
    – Zac67
    Jan 29 at 16:41
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    @brucebanner wikipedia article has a very nice picture explaining all of this. I belive those 4 bytes you are missing are the checksum in the end.
    – Effie
    Jan 29 at 17:41
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    @brucebanner The frame is 64 bytes, but if you add preamble and SFD it's 72 bytes.
    – Zac67
    Jan 29 at 18:18

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