• Subject: Re: Confusion reigns supreme (TCP/IP)
  • From: Larry Bolhuis <lbolhui@xxxxxxx>
  • Date: Fri, 08 Oct 1999 00:17:50 -0400
  • Organization: Arbor Solutions, Inc

Jim,

> I am told that my ip range is x.x.x.112 Block Size: /28
> 
> So, I have 112-140.  My net mask is 255.255.255.240

  You are misreading the x.x.x.112/28.  This is cidr format and does not mean
.112 for 28 addresses but rather that of the x.x.x.112 when represented in 
binary
the first 28 bits are NETWORK the remaining 4 bits are HOST.  That means that
.112 is the network (in binary x.x.x.01110000).  You have available addresses up
to .126 (x.x.x.01111110) and .127 is the broadcast address (x.x.x.01111111).

> wouldn't a mask of 255.255.255.240 mask off all bits but the last 4?

  Yup.

> so wouldn't my range be from 0 to 15 with 0 and 15 being unusable?

  Yup again. But remember it's 112+0 to 112+15 (112 to 127)

> Or is there something I don't understand about net masks?  (I wouldn't
> doubt that).
 
  A subnet mask of 255.255.255.240 says the same thing as the /28.  Lots of 
ISP's
use the /28 cause it's quicker to write and is actually more easily understood.
(Quick how many bits are on in 224? How about 248?)

> Anyway, thanks for the help, but there is something funny going on, I think,
> with the AS/400 (or V3R7M0 in particular), IP address, netmasks and the
> internet.

  The funny part is not the AS/400.  It is behaving as I would expect it to
except the part about NT and the router responding to PING. It is very strange
that if your NT box and Router each had the 255.255.255.240 mask that the AS/400
with a .139 address could ping them. Despite the fact that they are on the same
physical wire they would have been on different IP subnets and as such should 
not
have been able to communicate.  If the AS/400 subnet was set at 255.255.255.0
then the NT box may have responded (incorrectly) but any router worth it's
electrons shouldn't have.

  - Larry

Jim Langston wrote:


-- 
Larry Bolhuis         | What do You want to Reload today?
Arbor Solutions, Inc  | Don't throw your PC out the window,
(616) 451-2500        |  throw WINDOWS out of your PC.
(616) 451-2571 -fax   | Two rules to success in life:
lbolhui@ibm.net       | 1. Never tell people everything you know.
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