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ARP.md

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Address Resolution Protocol

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  • There is no IP header.
  • Type of 0x0806 is ARP.
  • ARP messages are variable in length. IPv4/Ethernet ARP message is 28 bytes.

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  • HTYPE - L2 protocol type. Value 1 is Ethernet.
  • PTYPE - L3 protocol type. Value 0x0800 (0d2048) is IPv4.
  • HLEN - Length (byte) of L2 address. Value 6 is MAC address.
  • PLEN - Length (byte) of L3 address. Value 4 is IPv4.
  • OPER - Type of ARP message. 1 is for request, 2 is for reply.
  • SHA - Sender's MAC address.
  • SPA - Sender's L2 address.
  • THA - Intended receiver's L2 address.
  • TPA - Intended reveiver's L3 address.

Proxy-ARP

Case #1 - PC1 can ping PC3/PC4 even whitout default gateway configured (Misconfigured Network Mask)

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Case #2 - R1 G0/0 is on behalf of 192.168.34.0/24 to reply corresponding ARP requests

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