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The logical network

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• Logical netwrk

  • Logical network:

    Logical network = a network created by the use of software

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  • More accurate definition:

    Logical network = a network that we create (using software) on top of multiple physical network The logical network will use the physical networks to: Transmit logical network messages (= packets)

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• Terminology for the Logical network
  • Packets:

    Packet = a message that is transmitted between nodes in a logical network Note: The logical network use use the physical network to transmitt messages !!!

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  • ISO level:

    The logical network is layer 3 in the ISO reference model:

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• Logical and virtual networks
  • Virtual network:

    Virtual network = a network that does not exists physically but created by software Logical networks are virtual networks !!!

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• The Internet

  • Fact:

    The Internet is a virtual network (created by software !!!)

    Case in point:

    You cannot buy a network card that uses IP addressess

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  • Fact:

    There are many virtual stuff that are pretty real...

    Example:

    You can even sell virtual islands for real money:

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• Structure of Internet packets

  • Complete structure of an IPv4 packet:

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  • For this discussion, we use an abreviated structure of an IP packet:

    4 bytes 4 bytes 32 <= 64K +-------+--------+-----+----------------+ | SrcIP | DestIP | CRC | Data | +-------+--------+-----+----------------+

    For now, all we need to know is:

    An IP packet has: a source IP address        and          a destination IP address

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    Note:

    An IP packet is called: a Layer 3 "frame" (because the IP layer is a Network layer (= layer # 3 in ISO model)        A Layer 2 frame is: a frame that is processed by a Datalink layer (layer 2) (Layer 2 is the physical network !!!)