• Subject: Re: Confusion reigns supreme (TCP/IP)
  • From: Jim Langston <jlangston@xxxxxxxxxxxxxxxx>
  • Date: Fri, 08 Oct 1999 09:48:34 -0700
  • Organization: Conex Global Logistics Services, Inc.

Oh!  I got it now.  The thing is, it is working exactly the way I thought it 
worked.

I think it was just a matter of not being able to see the bit for the bytes.  I 
just
had
to bit it out and it makes perfect sense.

I forgot the mask doesn't tell me what my range is, but the combination of my IP
address and my mask does.  I forgot that a lot of people have the same netmak
112 is:
128 64 32 16 8 4 2 1
  0    1   1   1  0 0 0 0
240 is:
  1    1   1   1  0 0 0 0
Take an ip address in my range, say 115 which is:
  0    1   1   1  0 0 1 1
To see if an IP gets "through" I would AND the other IP to my mask.  Say 116:
  0    1   1   1  0 1 0 0   - 116
  1    1   1   1  0 0 0 0   - 240
-------------------------
  0    1   1   1  0 0 0 0   = 112, so is good (same when I mask my own IP)

Take another one, like  161:
  1    0   1   0  0 0 0 1 - 129
  1    1   1   1  0 0 0 0 - 240
--------------------------
  1    0   1   0  0 0 0 0 = 160, not 112 so is no good.

Regards,

Jim Langston

Larry Bolhuis wrote:

> 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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