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Chapter 3 - PowerPoint PPT Presentation

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Chapter 3. Address Resolution Protocol (ARP). ARP. ARP is a protocol that is part of the TCP/IP protocol suite Responsible for translating a logical IP address into a hardware address. Hardware Address. Consists of six two-digit hexadecimal characters separated by hyphens

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Chapter 3

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chapter 3

Chapter 3

Address Resolution Protocol (ARP)

  • ARP is a protocol that is part of the TCP/IP protocol suite
  • Responsible for translating a logical IP address into a hardware address
hardware address
Hardware Address
  • Consists of six two-digit hexadecimal characters separated by hyphens
  • Example: 00-E0-98-00-DE-D5
  • Pre-assigned to network interface cards when they are manufactured
  • Also called:
    • NIC address
    • MAC address – most common term
    • Physical address
    • Ethernet address
hardware address cont
Hardware Address – Cont.
  • Hardware address is unique
  • Used to identify source and destination
  • No good in identifying what network the host is located on
  • Difficult for humans to remember
ip addresses
IP addresses
  • Logical address that specifies what network the host is on as well as identifies a specific host on the network
  • Used by TCP/IP protocol suite
  • Example:
  • 32 bits in length
  • Some bits specify the network segment
  • Subnet mask is used to determine which bits specify the network
  • Example subnet:
ip conversion
IP Conversion
  • When a packet of data is being encapsulated on the source computer, the destination’s hardware address must be included in the packet header.
  • ARP is responsible for determining the hardware address of the destination before the frame can be sent out
  • ARP determines the MAC address of the destination from the destination’s IP address
  • This process is called resolving the address
host initialization
Host Initialization
  • Each host must initialize itself on the local network
  • The host sends out an ARP broadcast containing its IP address and its hardware address to all hosts on the local network
local destination host vs remote destination host
Local Destination Host Vs. Remote Destination Host
  • The sending host can determine if the frame is being sent to a local host or a remote host
  • This can be determined from the destination’s IP address
  • Example:
    • Source IP:
    • Destination IP:
    • Subnet Mask:
    • Local destination in this example
resolution on a local network
Resolution on a Local Network
  • ARP cache on the host sending the frame is checked to see if the destination IP address is found. If so, the specified MAC address is used
  • If no mapping is found in the local ARP cache, an ARP request is sent out
  • This request asks: “Who does this IP address belong to and what is your hardware address?”
  • This ARP request is sent out as a broadcast frame which goes to each host on the hetwork
local arp resolution cont
Local ARP Resolution – Cont.
  • Each host receiving the ARP request checks the IP address to see if it matches theirs
  • If the IP address does not match, the request is ignored
  • If the IP address does match, an ARP reply is sent back to the source host that originated the ARP request
  • The ARP reply can be sent back because the source IP and hardware address are contained in the ARP request
local arp resolution cont1
Local ARP Resolution – Cont.
  • The ARP cache on both machines is updated with the correct IP and corresponding hardware addresses
  • Communication can now be established between the two hosts
remote arp resolution
Remote ARP Resolution
  • In this case, the destination is located on a remote network instead of the local network
  • The destination will have to be reached through one or more routers
  • ARP must resolve the address of each router that stands between the source and destination
  • Each stop through a router is called a hop
  • ARP adds the hardware address for the first router when sending the frames out
remote arp resolution cont
Remote ARP Resolution – Cont.
  • The IP address of the destination is determined to be remote
  • The routing table on the source is checked to see if a known route exists to the network that the destination host is on
  • If a route is found,the ARP cache is checked to see whether the mapping to the router that’s needed to reach the destination is in place
  • If no route is found, the source machine checks its ARP cache for the default gateway IP address and corresponding MAC address-ARP resolution may be needed here for default gateway
remote arp resolution cont1
Remote ARP Resolution –Cont.
  • The source sends the destination IP address to the default gateway (router) in order for the router to determine which network the destination is on
  • The router determines whether the IP address of the destination is local or remote
  • If the destination is remote, the router consults its own routing table for a route to the network where the destination node is found
  • The router also consults its own ARP cache to determine the hardware address of the next router
remote arp resolution cont2
Remote ARP Resolution – Cont.
  • The source host was on a remote network, so the destination host has to consult its own routing table for a path back to the source.
  • It also must consult its own ARP cache as well to get the hardware address of the router that will be used to get the packet back to the source
  • The packet travels back through the routers to the original network where the source host is found
  • Arp –a
  • Ping
  • Ipconfig /all