Re: Separate IP address for VPC and MAC

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From: David Newman (dnewman_at_networktest.com)
Date: 12/03/04 

Date: Thu, 02 Dec 2004 18:03:22 -0800

On 12/1/04 11:20 PM, in article
1go4lo3.2bl0g51hk8mwyN%neillmassello@earthlink.net, "Neill Massello"
<neillmassello@earthlink.net> wrote:

>
>> Also, I still want to be able to ping the MAC from VPC when there is
>> no router present. How can this be done?
>
> I'll have to pass on that one, as I don't know enough about the
> relationship between hardware and IP addresses, ARP, and ping to give a
> halfway knowledgeable answer. I can only wave my hands and suggest that
> the switch (in your router) can do hardware routing of ping packets back
> to the MAC from which they came, but that there's no provision for such
> a loop back from one IP to another on the same machine. It might be
> interesting to see if you get the same results by plugging the Mac's
> Ethernet cable into a simple switch, not a TCP/IP router.

Router vs. switch does not matter if both machines are on the same subnet,
as in this case.

Basic ARP on Ethernet (and WiFi) goes like this:

1. Host 1 sends out an ARP request asking for the MAC address associated
with Host 2's IP address. This ARP request message has a unicast destination
IP address (that of Host 2), but a broadcast destination MAC address.

The scope of the broadcast domain is the IP subnet. Thus, the broadcast ARP
request is seen by all hosts on the IP subnet to which Host 1 belongs.

2. If Host 2 is on the local IP subnet, Host 2 should send an ARP response
message to Host 1 with its MAC address. The ARP response is unicast, not
broadcast.

3. If Host 2 is not on the local IP subnet, the default gateway (the router)
will send an ARP response message with its MAC address. Essentially, this
means "send any traffic not on this subnet to my MAC address, and I'll take
it from there."

I have seen broken equipment violate these very basic rules, but this is how
ARP is supposed to work.

For further reading:
http://www.ietf.org/rfc/rfc826.txt

Regards,
David Newman
 

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