I was reviewing some old stuff and came across this one.
>The only exception to this is if you define the DG [default gateway]
for a
>device as its own IP. In this case, the machine will issue an arp
request
>for all destinations. ...
>If the destination being arped for is on the same physical
>LAN/VLAN, it will see the arp request for its MAC,
> but will ignore the request since it will recognize that the
> requesting station is on a different IP subnet.
> (I've done this in the lab, and this is what happens)
That's interesting, since I've done this to configure network devices
that
start with some funky, initial IP address like 192.0.0.192.
I simply (on my Win95 PC)
. remove my DG
. add new DG to my IP address
. then I can connect to 192.0.0.192
There is no Proxy ARP going on.
I've also tested it on Linux and works the same way.
RFC826 does not mention any matching of addresses.
Further, how could it really know whether it were on a different
subnet? --
there is no subnet mask being passed in ARP packets. The best it could
do
would be to make some kind of classful assumption, which would be bad.
So it seems to me that, if your device "ignore[ed] the request", then it
appears to have been an implementation bug.
-------------------------------------------------
Tks | <mailto:[EMAIL PROTECTED]>
BV | <mailto:[EMAIL PROTECTED]>
Sr. Technical Consultant, SBM, A Gates/Arrow Co.
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
=================================================
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Kent Hundley
Sent: Friday, July 21, 2000 2:55 PM
To: 'Karen E Young'; 'jeongwoo park'; [EMAIL PROTECTED]
Subject: RE:
Actually, the IP addresses matter very much.
It doesn't matter if you have 2 devices on the same LAN/VLAN, if their
IP
addresses are not in the same IP subnet, they will need a router to talk
to
each other. For example, if you have a station with IP address 10.1.1.1
and
another station with IP address 192.168.1.1, those 2 devices would need
a
router to talk to each other.
When an IP device needs to reach another IP device, it makes a
determination
based on its IP address and its subnet mask as to whether or not the
other
device is on the same subnet it is on. If it is, it will issue an arp
request. If it is not, it will send the packets to its DG.
If you have contiguous subnet ranges, you could play some games with the
masks, but then all you are doing is putting your devices on the same
subnet
for an IP address/subnet mask perspective, which doesn't change the fact
that a device will only issue an arp for a destination that is on its
own
subnet.
The only exception to this is if you define the DG for a device as its
own
IP. In this case, the machine will issue an arp request for all
destinations. If the router has proxy-arp turned on, it will respond
with
its MAC address. If the destination being arped for is on the same
physical
LAN/VLAN, it will see the arp request for its MAC, but will ignore the
request since it will recognize that the requesting station is on a
different IP subnet. (I've done this in the lab, and this is what
happens)
It would be possible to hard-code mappings in each devices arp table and
install static routes for each others subnet and get them to talk to one
another without an intervening router, but this is beyond the scope of
the
original question.
The bottom line is that under normal operating circumstances you need a
router for 2 IP devices that are not in the same IP subnet to talk to
one
another, regardless if they are physically on the same LAN/VLAN.
-Kent
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Karen E Young
Sent: Friday, July 21, 2000 6:57 AM
To: jeongwoo park; [EMAIL PROTECTED]
Subject: Re:
No, you don't need a router. A node is determined to be a member of a
VLAN
by their MAC address. Layer 2 rather than Layer 3 remember? The router
is
only needed to deal with packets destined for anything outside the VLAN.
The
IP addresses don't matter.
Karen E Young
*********** REPLY SEPARATOR ***********
On 7/20/2000 at 2:53 PM jeongwoo park wrote:
:HI all
:I have a question.
:Cisco recommends that there be one-to-one relationship
:between ip subnets and Vlans.
:When the number of devices on a Vlan exceeds the
:number of host ip addresses per configured subnet,
:more than one subnet can exit on a Vlan.
:Having said that, my question;
:There are two subnets in a Vlan. Do we need a router
:to interconnect these two subnets?
:I know that we need a router to interconnect two
:different Vlans.
:
:Thanks.
:
:jeongwoo
:
:
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