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Re: traffic filtering

  • From: Joe Abley
  • Date: Tue Jan 22 15:55:49 2002

On Tue, Jan 22, 2002 at 01:57:07PM -0600, J.F. Noonan wrote:
> 
> On Tue, 22 Jan 2002 at 12:34pm Joe Abley wrote:
> 
> > On Mon, Jan 21, 2002 at 05:53:16PM -0500, Stephen Griffin wrote:
> > > I'm curious about how many networks completely filter all traffic to
> > > any ip address ending in either ".0" or ".255".
> >
> > I heard recently that Windows 2000 will refuse to send packets
> > to addresses with the least-significant octet 255, if the most-
> > significant octet indicates the address lies in a pre-CIDR class
> > C. So, for example, 192.168.0.255 would be unreachable from a
> > windows 2000 machine, regardless of the fact that it might be
> > a legitimate host numbered within 192.168.0.0/23.
> 
> Not true.  M$ is guilty of many evil things, but not this one.

I just tried this. This is not exhaustive. I may well have made
some kind of some screw-up. Interpret as you will. Contents may
have settled in transit.

  NetBSD 1.5.2 i386         FreeBSD 4.5-PRERELEASE
    |                         |
 ---+------------+------------+----
                 |
               Win2k SP3

I configured the following addresses:

  NetBSD: 192.168.0.1/23, 192.168.0.255/23
  FreeBSD: 192.168.0.20/23
  Win2k: 192.168.0.30/23

FreeBSD box can ping 192.168.0.1. FreeBSD box can ping 192.168.0.255.
NetBSD box can ping 192.168.0.20. NetBSD box can ping 192.168.0.30
(tcpdump shows the NetBSD box is using a source of 192.168.0.1 for
these pings).

Win2k box can ping 192.168.0.1. Win2k box can ping 192.168.0.20.
Win2k cannot ping 192.168.0.255:

  C:\>ping 192.168.0.255

  Pinging 192.168.0.255 with 32 bytes of data:

  Request timed out.
  Request timed out.
  Request timed out.
  Request timed out.

  Ping statistics for 192.168.0.255:
      Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
  Approximate round trip times in milli-seconds:
      Minimum = 0ms, Maximum =  0ms, Average =  0ms

  C:\>

NetBSD box is receiving the requests, however, and replying to them.
tcpdump shows:

  15:30:39.753003 192.168.0.20 > 192.168.0.255: icmp: echo request
  15:30:39.753307 192.168.0.255 > 192.168.0.20: icmp: echo reply
  15:30:41.228742 192.168.0.20 > 192.168.0.255: icmp: echo request
  15:30:41.229053 192.168.0.255 > 192.168.0.20: icmp: echo reply
  15:30:42.230249 192.168.0.20 > 192.168.0.255: icmp: echo request
  15:30:42.230555 192.168.0.255 > 192.168.0.20: icmp: echo reply
  15:30:43.231735 192.168.0.20 > 192.168.0.255: icmp: echo request
  15:30:43.232046 192.168.0.255 > 192.168.0.20: icmp: echo reply

So, the Windows box seems to behave differently when dealing with
the particular address ending in 255 that I tried.

I guess the rule of thumb when numbering devices which need to
coexist with Windows is "avoid 255".


Joe




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