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network and internet - قالب بسته های ARP - RARP

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قالب بسته های ARP - RARP

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قالب بسته ARP

The above fig shows the ARP format used , below is the explanation of each field:

Hardware type
Each data link layer protocol is assigned a number used in this field. For Ethernet it is 1.

Protocol type
Each protocol is assigned a number used in this field. For example, IPv4 is 0x0800.

Hardware length
Length in bytes of a hardware address. Ethernet addresses are 6 bytes long.

Protocol length
Length in bytes of a logical address. IPv4 addresses are 4 bytes long.

Operation
Specifies the operation the sender is performing: 1 for request, and 2 for reply.

There are actually four types of ARP messages that may be sent by the ARP protocol. These are identified by four values in the "operation" field of an ARP message. The types of message are

1.ARP request
2.ARP reply
3.RARP request
4.RARP reply

Sender hardware address
Hardware address of the sender.


Sender protocol address
Protocol address of the sender.

Target hardware address
Hardware address of the intended receiver. This field is zero on request.

Target protocol address
Protocol address of the intended receiver.

ARP Function explained

ARP is used in four cases when two hosts are communicating


1.When two hosts are on the same network and one desires to send a packet to the other
2.When two hosts are on the different networks and must use a gateway or router to reach the other host
3.When a router needs to forward a packet for one host through another router
4.When a router needs to forward a packet from one host to the destination host on the same network


When an ARP response arrives, the receiver inserts a binding into an ARP cache so that it can be used for further packets. The oldest entry is removed if the table is either full or after an entry has not been updated recently. When an ARP request arrives, the receiver checks if it has the senders protocol address in the cache; if so, then the receiver updates the cache entry with the sender's binding. After a host replies to an ARP request, it adds the sender's binding to the cache - if a message travels from one host to another, then a reply will often travel back.

To understand this further lets see how an ARP actually works

ARP works by broadcasting the packet to all hosts attached to an Ethernet network. The packet contains the IP address the sender is interested in communicating with. The target machine, recognizing that the IP address in the packet matches its own, returnsan answer. Hosts actually keep a cache of ARP responses

 

قالب بسته های RARP

 


نوشته شده در : یکشنبه 7 آذر 1389  توسط : حسین جمالی.    نظرات() .

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