ARP question

[summoner]

I am involved in a dispute, hopefully someone here can settle this. Here is the scenario.

-Basic hub
-router with 2 interfaces connected to the hub (50.50.50.1 and 60.60.60.1)
-2 PCs connected to the hub (50.50.50.2 and 60.60.60.2)

If 50.50.50.2 wishes to communicate with 60.60.60.2, is their any ARP communication directly between the 2? My assumption is that both system go directly to the default gateway. If any ARP communication occurs, then it will between the PC and its respective default gateway, but not between the 2 pcs on different networks but same data-link network. Thanks in advance.

 

[tcvander]

Correct PC1 (50.2) will know (based on his IP address and mask) that PC2 (60.2) is on a different subnet, so PC1 will ARP for it's default gateway and then send all frames going to PC2 to the MAC address of his default gateway Todd VanDerwerken, CCNA, CCDA
Technical Consultant

http://www.ciscolinks.net

"If at first you don't succeed...then sky diving isn't for you!"

 

[summoner]

Thank you for the quick response. My friend insisted that 50.50.50.2 will talk directly to 60.60.60.2. Just to drive the point home...there will be no direct ARP communication between 50.50.50.2 and 60.60.60.2...only to their respective gateways.

 

[jimbopalmer]

If you chose a VERY permissive subnet mask, say 128.0.0.0, then these two devices would know they were on the same subnet, and talk directly. Any subnet mask like 255.n.n.n would put these in different subnets. n = 0 to 255
I tried to remain child-like, all I acheived was childish.

 

[ChrisAC]

128.0.0.0 would be an invalid subnet mask anyway so they are always going to be in different subnets so they will have to talk to the gateway in order to communicate.

If your friend doubts it then take the router out of the equation and see if the two machines can talk to each other. We all know what the result will be but at least you could wear a smug grin ;-)

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