arp and real communication

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  • 7/31/2019 ARP and Real Communication

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    Computer Networks

    ARP and Real Networking

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    1. IP Addresses and IP Addressing Rules

    What is a Physical Segment?

    ARP

    Local or Remote

    The Local or Remote Algorithm

    2.To Obtain the MAC Address From A Local Host:

    3. To Obtain the MAC Address From A Remote Host:

    4. What Breaks It!

    ARP and Real Communication

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    IP Address

    Unique Device (Host) Address

    Identifies which Portion of IP Address is the Network ID andWhich Portion is the Host ID

    IP Address of the Router on the Same Physical Segment

    Subnet Mask

    Default Gateway152.107.102.7 (IP Address)

    255.255.0.0 (Subnet Mask)

    152.107.102.1 (Default Gateway

    IP Addresses

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    Each Device (Host) Needs at Least One Unique IP Address

    All Devices on the Same Physical Segment Share aCommon Network ID (Subnet Mask)

    Each Physical Segment Has a Unique Network ID (SubnetMask)

    What is Physical Segment

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    The portion of the network that you can retrieve informationfrom by using a broadcast packet!

    Ignore Repeaters, Bridges, or Switches

    Forward Broadcasts

    Everything (all devices) --

    Out a port of a router

    Between two routersRouters Dont Forward Broadcasts

    IP Addressing Rules

    All Devices on the Same Physical Segment Share a Common

    Network ID

    Each Physical Segment Has a Unique Network IDs

    Broadcasts

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    Address Resolution Protocol

    RFC 826

    Maps a MAC (Hardware) Address to a Destinations Hosts orRouters IP Address

    Obtains MAC Address From Cache or With a Local Broadcast

    The MAC Address is Used in the Address Field of the Data-Link Frame (Ethernet, Token Ring, etc.)

    ARP

    http://info.internet.isi.edu/in-notes/rfc/files/rfc826.txthttp://info.internet.isi.edu/in-notes/rfc/files/rfc826.txt
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    Local The Destination Device/Host is on the Same

    Physical Segment/Subnet

    Remote The Destination Device/Host is on a Different

    Physical Segment/Subnet

    Local or Remote

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    Local or Remote algorithm

    To Setup a TCP Connection:

    IP Address of Destination Computer is Obtained (DNS,WINS, Broadcast, etc.)

    IP Address of Destination is Analyzed to Determine if it

    is Local or RemoteThe Source Host Uses Its Subnet Mask to Determine the NetworkID of the Destination Hosts IP Address

    The Source Host Compares the Destinations Network ID to Its

    Network ID

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    To obtain MAC address

    If the Destination Hosts Network ID is the Same as theSource Hosts Network ID; the Destination is Local (Onthe Same Physical Segment/Subnet)

    The Source Host Checks its ARP Cache For a Cached MACAddress of the Destination Host

    If the Destination Hosts MAC Address is Not in Cache, A

    Local Broadcast is Initiated to Obtain the Destinations HostMAC Address

    The Returned MAC Address is Cached for Reuse

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    If the Destination Hosts Network ID is Different Fromthe Source Hosts Network ID; the Destination Host isRemote (On a Different Physical Segment/Subnet)

    The Source Host Checks Its Local Route Table for a SpecialRoute to the Network of the Destination Host

    If There is a Special Route, The Source ComputerChecks its ARP Cache For a Cached MAC Address of theRouter Associated With the Route in the Local Route Table

    If the Routers MAC Address is Not in Cache, a Local

    Broadcast is Initiated to Obtain the Routers MAC Address

    The MAC Address is Cached for Reuse

    To obtain MAC address for Remote Host

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    MAC Address for Remote Host

    If There is No Special Routein the Source Hosts LocalRoute Table, The Source Computer Checks its ARP Cache For

    a Cached MAC Address of the Default Gateway If the DefaultGateways MAC Address is Not in Cache, A Local Broadcast isInitiated to Obtain the Default Gateways MAC Address

    The MAC Address is Cached for Reuse

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    What breaks it ??

    IP Address Entered Incorrectly

    All Hosts on the Same Physical Segment Must Have the SameNetwork ID

    Each Physical Segment Requires a Unique Network ID

    Subnet Mask Incorrectly

    Break Local/Remote AlgorithmDefault Gateway Entered Incorrectly

    Can Communicate Outside Local Segment

    Incorrect Entries in Local Route Table

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    ARP

    Maps a MAC (Hardware) Address to an IPAddress

    Obtains MAC Address From Cache or With aLocal Broadcast

    Works Automatically if IP Addresses are

    Correct!