MESSAGE
| DATE | 2016-04-13 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [Hangout-NYLXS] confused as to arp tables
|
This is the snippet of text from the book
Now let’s examine how a host on Subnet 1 would send a datagram to a host
on Subnet 2. Specifically, suppose that host 111.111.111.111 wants to send an IP
datagram to a host 222.222.222.222. The sending host passes the datagram to its
adapter, as usual. But the sending host must also indicate to its adapter an appro-
priate destination MAC address. What MAC address should the adapter use? One
might be tempted to guess that the appropriate MAC address is that of the adapter
for host 222.222.222.222, namely, 49-BD-D2-C7-56-2A. This guess, however,
would be wrong! If the sending adapter were to use that MAC address, then none
of the adapters on Subnet 1 would bother to pass the IP datagram up to its net-
work layer, since the frame’s destination address would not match the MAC
address of any adapter on Subnet 1. The datagram would just die and go to data-
gram heaven.
If we look carefully at Figure 5.19, we see that in order for a datagram to go
from 111.111.111.111 to a host on Subnet 2, the datagram must first be sent to the
router interface 111.111.111.110, which is the IP address of the first-hop router
on the path to the final destination. Thus, the appropriate MAC address for the
frame is the address of the adapter for router interface 111.111.111.110, namely,
E6-E9-00-17-BB-4B. How does the sending host acquire the MAC address for
111.111.111.110? By using ARP, of course! Once the sending adapter has this
MAC address, it creates a frame (containing the datagram addressed to
222.222.222.222) and sends the frame into Subnet 1. The router adapter on Sub-
net 1 sees that the link-layer frame is addressed to it, and therefore passes the
frame to the network layer of the router. Hooray—the IP datagram has success-
fully been moved from source host to the router! But we are not finished. We still
have to move the datagram from the router to the destination. The router now has
to determine the correct interface on which the datagram is to be forwarded. As
discussed in Chapter 4, this is done by consulting a forwarding table in the router.
The forwarding table tells the router that the datagram is to be forwarded via
router interface 222.222.222.220. This interface then passes the datagram to its
adapter, which encapsulates the datagram in a new frame and sends the frame
into Subnet 2. This time, the destination MAC address of the frame is indeed the
MAC address of the ultimate destination. And how does the router obtain this
destination MAC address? From ARP, of course!
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
So here is the confusion. It reaches the router and it now needs to use
arp to obtain the destination MAC for the packet from the second network.
There two interfaces are not on the same subnet. Moreover, the desination host
with a second subnet is usually on yet a third! subnet. How will it ever
get a distination MAC?
--
So many immigrant groups have swept through our town
that Brooklyn, like Atlantis, reaches mythological
proportions in the mind of the world - RI Safir 1998
http://www.mrbrklyn.com
DRM is THEFT - We are the STAKEHOLDERS - RI Safir 2002
http://www.nylxs.com - Leadership Development in Free Software
http://www2.mrbrklyn.com/resources - Unpublished Archive
http://www.coinhangout.com - coins!
http://www.brooklyn-living.com
Being so tracked is for FARM ANIMALS and and extermination camps,
but incompatible with living as a free human being. -RI Safir 2013
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