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Regarding Pc to PC ping via hub

charlesdaniel

Hi Team,

I have attached the image.

My question is

1.Will it pinng eventhough it is in diff subnet.

2.As hub doesnt know anything abt networks,will it forward and ping works?

Please clarify..

1.jpg

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Comments

Vask3n

You'd need a default gateway/device that can route somewhere in the mix if you want to pass traffic from one network to another.

fredrikjj

Yes, if you use proxy ARP on the hosts, but it doesn't have anything to do with the hub per se. It probably depends on the implementation, but if the host doesn't have a default gateway it will ARP for any destination. That ARP will reach the other host which will respond with its own MAC address because it has proxy ARP enabled.

PS.
You could also set static arp entries on the hosts.

charlesdaniel

Hi Vask3n

Thank you for reply..i cant enable any default g/w in pc as no extra setup is there in b/w hub and pcs..

so it wont ping unless and untill there is static route known both of the devices to know each other..isnt it?

Dieg0M

Since when can a hub do proxy ARP?

Edit: Fredrik edited his post to clarify.

charlesdaniel

yes..how hub will do proxy arp?

charlesdaniel

thanks,how hub will do proxy arp as it is a phy device..also no extra setup is there

fredrikjj

Where do I say that the hub will do proxy arp?

Heero

Dieg0M wrote: »

Since when can a hub do proxy ARP?

I think what he meant was that if PC1 realizes 2.2.2.2 is not on the same subnet and PC1 doesn't have a default gateway, it could potentially send an arp request for an IP that is on a different subnet and then 2.2.2.2 could respond because it is on the same broadcast domain. Kind of like proxy arp, but you don't have a device that is actually doing the proxy arp because the real host is on the same broadcast domain.

Dieg0M

I am not sure to understand, who is doing the proxy arp? As far as I know, a host can't do proxy arp for itself unless it is has 2 NIC's and running some kind of application that can simulate proxy arp.

fredrikjj

Dieg0M wrote: »

I am not sure to understand, who is doing the proxy arp? As far as I know, a host can't do proxy arp for itself unless it is has 2 NIC's and running some kind of application that can simulate proxy arp.

I tested this on IOS before I wrote the first post. I have no idea if say a Windows 7 machine will do the same thing, which is why I said:

It probably depends on the implementation..

Here is how I think it works:

Host 1 sends an ARP for 2.2.2.2. Host 2 responds because it's running proxy ARP on that interface. Host 1 is now able to encapsulate the frame with the MAC address received in the ARP reply from host 2. Host 1 sends the ICMP echo request or whatever it is to host 2. Host 2 ARPs for host 1's MAC address. Host 1 responds because it's running proxy ARP. Both hosts now have the other host in their ARP cache and host 2 can send the echo reply.

Dieg0M

Ok, Fredrik has been spending too much time in his lab

The truth is I have not seen this in a production environment and I believe you would have to make special modifications for the host to answer ARP requests that are on the same interface as the ARP request came from. In theory, it is possible and after a quick look on the web I have seen some people posting kernel patches for unix platforms to be able to accomplish this.

Vask3n

Thought it would be cool to read more on this, Cisco has an official documentation Proxy Arp

"Proxy ARP is the technique in which one host, usually a router, answers ARP requests intended for another machine.

Proxy ARP can help machines on a subnet reach remote subnets without the need to configure routing or a default gateway"

http://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/13718-5.html

charlesdaniel

Thanks for the clarification

charlesdaniel

Thanks Fredrik

charlesdaniel

Thanks all