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If I ping to another pc in my network I send a packet that consists out of an ICMP part, a IPv4 part and a Ethernet II part. I can see this using the packet analyser 'wireshark'. Now the Ethernet II part consists of three parts again:

  1. The source (The MAC address of the network interface controller of the pc that sent the ping)

  2. The destination (The MAC address of the network interface controller of the pc that received the ping)

  3. The type (What is this?) It says Type: IP (0x0800)

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1 Answer 1

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The "Type" field in Ethernet II frames tells the OS what kind of data the frame carries – 0x0800 means that the frame has an IPv4 packet; there's a list of different EtherTypes.

This field is needed because there are many other protocols that go directly over Ethernet: for example, IPv6, IPX, ARP, AppleTalk...

This is explained in detail in the Wikipedia article about Ethernet frames, in particular the section about Ethernet frame types – Ethernet II ("DIX") frames have a "Type" field, but some networks (in particular the now-obsolete IPX) used to use IEEE 802.3 framing, which has "Packet size" instead, and uses separate 'LLC' (802.2) or even 'LLC'+'SNAP' headers between Ethernet header and protocol data.

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