arp and real communication
TRANSCRIPT
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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!