Posted by arlene

As just discussed, IP provides a logical hierarchical address space that makes routing data from one network to another a simple task. When the datagram arrives at the local subnet, however, another protocol comes into play. The Address Resolution Protocol (ARP) is used to resolve the IP address to the hardware, to the address of the workstation, or to another network device that is the target destination of the datagram. Whereas IP addresses are used to allow for routing between networks or network segments, ARP is used at the end of the road for the final delivery.

It is important to understand that when devices communicate directly on the local network segment (on the wire, so to speak), the actual address used to communicate between two devices, whether they are computers, routers, or whatever, is the built-in Media Access Control (MAC) address. In the case of two hosts on the same subnet, ARP can quickly resolve the correct address translations, and communications take place quickly and efficiently. When a router stands between two computers, the actual hardware address that the computer communicates with is the MAC address of the router, not of the computer that lies at the end of the connection. Using Ethernet as an example, when a datagram needs to be routed to another network or subnet, the computer sends the datagram to the default route, sometimes called the default gateway, which is the router that connects the network segment to the rest of the world (or the rest of the corporate network).

The router then consults its routing tables and decides on the next device that the packet needs to get to on its way to its destination. Sometimes this is simply a computer that is connected on another segment that is also connected to the router. Sometimes it is several more routers that the packet must pass through. However, when the packet finally reaches the network segment on which the target computer is located, ARP is used by the router to find out the MAC address of the computer that is configured with the IP address found inside the packet.

To get this MAC address, a computer or router will first send out a broadcast message that every computer on the local segment can see. This ARP message contains the sending computer’s own MAC address and also the IP address of the computer to which it wants to talk. When a computer recognizes its IP address in this broadcast packet, it sends a packet that contains its own MAC address back to the computer that originated the ARP message. After that, both computers know the MAC address of the other, and further transmissions take place using these hardware addresses.

The actual fields in the ARP broadcast frame are listed here:

• Hardware Type—This is a 2-byte field that identifies the kind of hardware used at the data- link layer of the sending computer.

• Protocol Type—This is a 2-byte field that specifies the protocol type of the address that the computer wants to translate to a hardware address.

• Hardware Address Length—Thisis a 1-byte field that specifies the length of the source and destination hardware address fields that will follow.

• Protocol Address Length—Similarly, this 1-byte field specifies the length of the source and destination protocol address fields that will follow in this packet.

• Opcode—This 1-byte field is used to determine the type of ARP frame.

• Sender Hardware Address—This variable-length field (as defined by the Hardware Address Length field) contains the sending computer’s hardware (MAC) address.

• Sender Protocol Address—Thisvariable-length field (as defined by the Protocol Address Length field) contains the sender’s protocol address—an IP address, for example.

• Target Hardware Address—This variable-length field (as defined by the Hardware Address Length field) contains the destination computer’s hardware (MAC) address.

• Target Protocol Address—This variable-length field (as defined by the Protocol Address Length field) contains the protocol address that the sender wants to resolve to a hardware address.

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