Address Resolution Protocol (ARP)

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Chapter 3

• Address Resolution Protocol (ARP)

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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

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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

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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

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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 118.14.126.57
• 32 bits in length
• Some bits specify the network segment
• Subnet mask is used to determine which bits
  specify the network
• Example subnet 255.255.255.0

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IP Conversion

• When a packet of data is being encapsulated on
  the source computer, the destinations 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

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ARP

• ARP determines the MAC address of the destination
  from the destinations IP address
• This process is called resolving the address

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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

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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 destinations IP
  address
• Example
• Source IP 118.14.126.17
• Destination IP 118.14.126.32
• Subnet Mask 255.255.255.0
• Local destination in this example

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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

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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

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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

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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

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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 thats
  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

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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

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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

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Utilities

• Arp a
• Ping
• Ipconfig /all