why arp is being used?

[kusichen]

Can somebody explain?
i mean routers find the network, on which our destination IP address resides. the datagrams sent one by one...

even if I assume that it is needed because of intra-network requirements. Why is the sending host getting the MAC-Address first , before sending the datagrams, and including this MAC Address within each datagram to be sent?
it is confusing...

 

[crazynoodle]

An Arp request on a local segment is a layer 2 broadcast. Routers by default do not forward broadcasts, so the router is sending back the interfaces mac address and this continues along the path to your layer 3 destination ip address.

 

[kusichen]

Salut crazy,
before doing any sort of connection, the sending host must get the MAC Address of the destination host. right? and this is via an ARP request. But for getting the MAC Address must somehow interact with the router, residing on the network/subnetwork to which our destination host belongs. This means --> we reached the host already without MAC Address....

After having gotten the MAC Address, then our sending host donot deal with ARP again. and attaching the MAC Address somewhere in the Header-Trailer of Data Link Level, sends to a router... too be sent to that far network

IP Address is enough, still I Think ..??

 

[crazynoodle]

Hello,

If you have a default gateway set on your pc and that is the router in question then I pretty certain you have the mac address of this interface in your arp cache

at a cmd prompt type arp -a

So in this case you would not need to send a arp request. But if you didnt have default gateway set you could configure the router to perform proxy-arp then it would pick up the broadcast and take care of it.

In anycase the mac address is needed for the endstations to process the packet, but true you will only arp for a mac address you do not know. And if this endstation is on another physical segment then everything is sent to the default gateway IP address/mac address.

That is my take on it.

~CN~

 

[ChrisAC]

kusichen,

Are you trying to understand why devices communicate using a MAC address or why ARP is used to discover the MAC address?

Chris.


**********************
Chris Andrew, CCNA, CCSA
chris@iproute.co.uk
**********************

 

[kusichen]

Hi route,
i mean, the data link and the media layers are hardwired. so MAC or such a scheme is logical. But it should go one by one.
host-sending----router1-R2...-routerN---host-destination
!---routerPI
!---routerupsilon

@host:I just need to know the MAC of router1.nothing else.
@router1:depending on DNS AND IP algorithm, I choose any of the possible neighboring routers.and just need to know,say, router2 MAC Address,
...
@routerN: i must know MAC of host destination.

 

[greg5149]

You are missing the whole layer 3 part of the equation in reference to the routing table in reference to the router.
The MAC or layer 2 is for the interfaces. The IP address will get you to the host's router as long as the route is learned via algorithim or via static route. The first and last routers will take care of the MAC (forwarding table) of the hosts.
Greg5149