ARP

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ARP:
1:The device driver software never looks at the destination IP address in the IP datagram.
2:Address resolution provides a mapping between the two different forms of addresses: 32-bit IP addresses and whatever type of address the data link uses.
3:arp -a, arp -d; sh arp, clear arp
ARP packets format:
   6B       6B  2B(ARP) 2B(Ethernet)   2B(IP)        1B            1B           2B    6B        4B    6B     4B
Des.MAC|Sou.MAC|Type|HardwareType|ProtocolType|HardwareAddressLen.|ProtocolAddressLen.|OP|SenderMAC|SenderIP|Tar.MAC|Tar.IP
               0x0806  0x0001          0x0800        0x06        0x04         
OP:ARP request(0x0001); ARP reply(0x0002); RARP request(0x0003); RARP reply(0x0004).
PAD:6+6+2+2+2+1+1+2+6+4+6+4=42, number of bytes of padding is 18B.
The work process of Switcher:(CAM table)
1:PCs and servers learn MAC-to-IP by arp ARP request and ARP reply;
2:Switcher learns MAC-to-IP and puts entry into CAM table by ARP request;
3:Switcher transmits packets by CAM table
Proxy ARP and Gratuitous ARP
Proxy ARP:
Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks. This fools the sender of the ARP request into thinking that the router is the destination host, when in fact the destination host is "on the other side" of the router. The router is acting as a proxy agent for the destination host, relaying packets to it from other hosts.
Gratuitous ARP:
It occurs when a host sends an ARP request looking for its own IP address. This is usually done when the interface is configured at bootstrap time.
# 2009-09-12 14:08
               
               
               

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