Address Resolution Protocol ARP

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Address Resolution Protocol (ARP)
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What is ARP?

• ARP is a protocol
• used by the Internet Protocol (IP) network layer
  protocol
• to find the physical address of a target on the
  same physical network when the host has only the
  IP address of the target

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Software network address and hardware network
address
Software Network Address
1. Network address managed by a network protocol
Example(1) IP address in Internet Protocol
(2) computer name in Microsoft
NetBEUI
2. Network address can be changed through OS
Hardware Network Address

• Network address hardwired in each NIC
• (assigned at a factory)

Example Ethernet MAC address
2. Can not be changed
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Why ARP is needed?

• NIC can understand ONLY hardware network address

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How ARP works?

• Host A wants to send data to host B
• Host A knows IP address of B (but not MAC address
  of B)

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ARP Packet Format

• Hardware type 1 for Ethernet
• Protocol type 0x0800 for IP
• HLEN length of hardware address, 6 for Ethernet
• PLEN length of protocol address, 4 for IP

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ARP Packet Format (continued)

• Operation
• 1 for ARP request
• 2 for ARP reply
• 3 for RARP request
• 4 for RARP reply

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ARP Cache

• ARP cache entry lt IP address MAC address TTLgt
• All hosts that receive the broadcast ARP request
  extract and store the hardware address
  corresponding to the IP address of the sender
• The sender stores the hardware address when it
  receives an ARP reply from the target
• A timeout is provide for each entry in the ARP
  cache. Normally 20 minutes
• The timeout for an entry restarts each time the
  entry is used