TCP IP Illustrated

6 arp sends an ethernet frame called an arp request

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Unformatted text preview: P request contains the IP address of the destination host (whose name is bsdi) and is the request "if you are the owner of this IP address, please respond to me with your hardware address." file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (2 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol Figure 4.2 Operation of ARP when user types "ftp hostname". 7. The destination host's ARP layer receives this broadcast, recognizes that the sender is asking for its hardware address, and replies with an ARP reply. This reply contains the IP address and the corresponding hardware address. 8. The ARP reply is received and the IP datagram that forced the ARP request-reply to be exchanged can now be sent. 9. The IP datagram is sent to the destination host. The fundamental concept behind ARP is that the network interface has a hardware address (a 48bit value for an Ethernet or token ring interface). Frames exchanged at the hardware level must be addressed to the correct interface. But TCP/IP works with its own addresses: 32-bit IP addresses. Knowing a host's IP address doesn't let the kernel send a frame to that host. The kernel (i.e., the Ethernet driver) must know the destination's hardware address to send it data. The function of file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (3 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol ARP is to provide a dynamic mapping between 32-bit IP addresses and the hardware addresses used by various network technologies. Point-to-point links don't use ARP. When these links are configured (normally at bootstrap time) the kernel must be told of the IP address at each end of the link. Hardware addresses such as Ethernet addresses are not involved. 4.3 ARP Cache Essential to the efficient operation of ARP is the maintenance of an ARP cache on each host. This cache maintains the recent mappings from Inter...
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