H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 13)

2
• Protocol type: The type of the protocol address to be mapped. The hexadecimal value 0x0800
represents IP.
• Hardware address length and protocol address length: Length, in bytes, of a hardware address
and a protocol address. For an Ethernet address, the value of the hardware address length field is
6. For an IP(v4) address, the value of the protocol address length field is 4.
• OP: Operation code. The type of the ARP message. The value 1 represents an ARP request and 2
represents an ARP reply.
• Sender hardware address: Hardware address of the device sending the message.
• Sender protocol address: Protocol address of the device sending the message.
• Target hardware address: Hardware address of the device the message is being sent to.
• Target protocol address: Protocol address of the device the message is being sent to.
Operation of ARP
If Host A and Host B are on the same subnet and Host A sends a packet to Host B, as shown in Figure
2 , the resolution process is:
1. Host A looks in its ARP table to see whether there is an ARP entry for Host B. If yes, Host A uses the
MAC address in the entry to encapsulate the IP packet into a data link layer frame and sends the
frame to Host B.
2. If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request using
the following information:
{
Source IP address and source MAC address: Host A’s own IP address and the MAC address
{
Target IP address: Host B’s IP address
{
Target MAC address: An all-zero MAC address
Because the ARP request is a broadcast, all hosts on this subnet can receive the request, but only the
requested host (Host B) will process the request.
1. Host B compares its own IP address with the target IP address in the ARP request. If they are the
same, Host B:
{
Adds the sender IP address and sender MAC address to its ARP table.
{
Encapsulates its MAC address into an ARP reply.
{
Unicasts the ARP reply to Host A.
2. After receiving the ARP reply, Host A:
{
Adds the MAC address of Host B to its ARP table.
{
Encapsulates the MAC address into the packet and sends it to Host B.