Address Resoultion Protocol Questions.

[SPYME.GOOGLE]

HI

If my router has two different networks connected to two ethernet interfaces and if a host in n/w 1 wants to communicate with a host in n/w 2 how the Address resolution protocol works in the below condition [Assume that all the arp tables of all the hosts and router are empty.]

1. THE DEFAULT GATEWAYS FOR ALL THE HOSTS IN BOTH THE NETWORKS ARE SET TO THEMSELVES.

My questions

1. Will the host in n/w 1 will arp for itself or will it arp for the router or will it arp for the destination. [Remember both the netwros are different]

Please explain the setp by step process. Many thanx

[sourabhagarwal]

Hi,

Gateway of host in n/w1 will do proxy ARP to reach hosts in n/w 2. Similarly, gateway of host in n/w 2 will do proxy ARP to reach hosts in n/w 1.

please browse below link to understand step by step process.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml

rate this post if it helps ...

[SPYME.GOOGLE]

Hi

Thanks for your reply. If the gateway of the host in n/w1 is the host itself then what will the host do. What i mean is if H1 in n/w1 gateway is H1 itsef and if H1 wants to send a packet to H2 in n/w 2 what will it do. Rememebr gateway of H2 in n/w 2 is H2 itself.

And the link you sent mentions about proxy ARP. Do you have any link which explains about the concept of Default gateway.

[sourabhagarwal]

in this case if H1 itself is a gateway and it want to reach H2, source MAC address will be H1s MAC add and destination MAC address will be MAC addr of H2 or MAC addr of intermediate gateway between H1 and H2 if any depending on the below cases.

1. If H1 is directly conected to H2, then when H1 will send ARP broadcast, H2 will send its MAC addr in ARP reply packet.

2. if there is a Layer 3 device acting as gateway between H1 and H2, then H1 will send ARP broadcast message and gateway of H1 will respond with ARP reply with its MAC address because L3 device won't forward broadcast packets to another network. H1 can use this MAC addr to build frame to send it across to H2.

below links can give you overview of default gateway concept.

http://www.microsoft.com/technet/community/columns/cableguy/cg0903.mspx

http://en.wikipedia.org/wiki/Default_gateway

http://www.inetdaemon.com/tutorials/internet/ip/default_gateway.shtml

hope this helps ..

[abhishek_nandavat]

Hi,

For detailed info on how ARP works, have a look at the following links-->

http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/arp.html

http://en.wikipedia.org/wiki/Address_Resolution_Protocol

For default gateway, look the following links-->

http://en.wikipedia.org/wiki/Default_gateway

http://www.inetdaemon.com/tutorials/internet/ip/default_gateway.shtml

Hope this helps!

Regards,

AbhisheK

Please rate all posts!!!

[leighharrison]

Hi there,

A default gateway is used on a pc when there is an ip address a network mask and the address wanted in not in that network. i.e:-

you have:-

192.168.1.1 255.255.255.0

and your pc wants to find 10.1.2.3. The address it wants to find is not on your local network, so it can't find it. This is where the default gateway comes in.

If the address is not on your network - send it to the default gateway. This is usually a router and will know where the network lives and will send the packet on its way.

If the default gateway is set to itself, then it will send the packet to itself. And if it didn't know where it was the first time, it won't know the second time!

The default gateway need to be a device that knows how to get to other networks.

Arp'ing will be doing nothing, as your pc will be asking for itself and it should know where it lives ;-)

Hope this helps to clarify,

LH

Please rate all posts

[SPYME.GOOGLE]

Hi

Thanx for al your replies. In i set the default gateway if H1 as H1 itself and H2 is in a different network then how does ARP work.

1. WIll H1 ARP for itself. If so what is the resolution for it.

2. How will H1 comes to know that only the router between the two networks can complete the process.