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arp and default gateway.

Mr.Ping

Hi guys am stuck....help!!

How does a workstation use arp to determine if a host its trying to communicate with is in the local or a remote network? (i would have thought it would do a logical and but the books seem to say am wrong)

secondly what is the role of the default gateway vis a vis arp?ie why would a workstation choose to send a packet to the default gateway rather than issue an arp broadcast??

Thirdly,if proxy arp is not enabled on a router connecting two devices,will an arp broadcast be dropped even if the router has a path to the destination network?

Hope someone can switch on the lights for me.Will really appreciate.

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Comments

EdTheLad

melo wrote:

Hi guys am stuck....help!!

How does a workstation use arp to determine if a host its trying to communicate with is in the local or a remote network? (i would have thought it would do a logical and but the books seem to say am wrong)

A workstation will examine is local ip address and mask, from this examination it will know the range of possible host addresses on the local segment.If the destination ip address is not within this range, it will use the default gateway.
To encapsulate the packet,the data is encapsulated with an ip header that contains the src and dest ip address.Then the packet gets encapsulated in a frame,so you need the src and dest mac address!! This is a problem, we dont know the destination mac address to this is why we use arp.
To send to a remote machine we used the default-gateway, to send to the DG we need the mac address of the default gateway,so the workstation first must arp the DG.

secondly what is the role of the default gateway vis a vis arp?ie why would a workstation choose to send a packet to the default gateway rather than issue an arp broadcast??

An arp broadcast is sent, this broadcast has a layer 2 broadcast address FF.FF.FF.FF.FF.FF and the layer 3 ip address of the default-gateway.

Thirdly,if proxy arp is not enabled on a router connecting two devices,will an arp broadcast be dropped even if the router has a path to the destination network?

Yes, if the arp broadcast doesnt have the destination ip address of the routers interface
it will be discarded when proxy arp is disabled.

Mr.Ping

Hi Ed,
thanx for that.I realise i was separating layer 2 from three but they work together!!
Let me digest that.

thanx alot.