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Re: [lwip-users] OT: Is the AutoIP range a special case IP address range


From: Simon Goldschmidt
Subject: Re: [lwip-users] OT: Is the AutoIP range a special case IP address range?
Date: Tue, 16 Nov 2010 08:01:34 +0100

David Empson <address@hidden>
>> OT: Is the AutoIP range a special case IP address range?Bill Auerbach
>> wrote:
>>   I just found out through testing that the PC with a NIC in the
>> 192.168/16 subnet (netmask 255.255.255.0) will ping and connect to devices 
>> on the
>> physical network in the AutoIP range (169.254/16).  This is on a WinXP PC.  I
>> find the ARP table contains AutoIP addresses and I can ping and open
>> connections to the device at the AutoIP address.
>> 
>>   Is this something that is expected to occur?

Yes, it's expected behaviour.

>>  Is it specific to Windows?

No, it's defined in RFC 3927.

>>  I've not come across any RFC that mentions that this 'bridging' should or
>> can occur.

As David said, no bridging required. Here's an excerpt from RFC 3927:
>>>> snip >>>>
1.8.  Communication with Routable Addresses

   There will be cases when devices with a configured Link-Local address
   will need to communicate with a device with a routable address
   configured on the same physical link, and vice versa.  The rules in
   Section 2.6 allow this communication.

   This allows, for example, a laptop computer with only a routable
   address to communicate with web servers world-wide using its
   globally-routable address while at the same time printing those web
   pages on a local printer that has only an IPv4 Link-Local address.
<<<< snap <<<<

Windows might use a 2nd IP address in the AutoIP range, but that's not 
necessary if AutoIP is implemented correctly in all devices: IP traffic to 
Link-Local address should mainly always be sent directly by using a local ARP 
request.

BTW: This does *not* work for me under Windows XP, where I have to manually 
configure a 2nd address in the AutoIP range. Which version of windows were you 
using, Bill and David?

Simon
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