In a routable protocol, every address needs two parts: a "network"
part, and an individual "host" part.  Think "area code" and "local
phone number".  IPX (Novell) and AppleTalk make these separate
fields; IP combines them into a single 32-bit value.
  The subnet mask is used when it is necessary to break out the
network and host parts of the address.  It identifies which of the 32
bits are the network part.  If you write the subnet mask out in
binary (eight bits per octet), you should see ones starting from the
left and changing to zeros somewhere in the middle.  Once they change
to zeros, there will be no more ones.  (Some modern software/devices
let you specify the number of ones, instead of spelling out the mask.
 e.g. 10.1.2.3/8 -- 8 ones is the same as a mask of 255.0.0.0.)

  So if you enter the subnet mask as 10.20.30.40, then in binary
that's

  00001010.00010100.00011110.00101000

which violates the "all ones from the left, and then all zeros" rule.
In contrast, 255.255.255.0 becomes, in binary

  11111111.11111111.11111111.00000000

which is much more reasonable.

  The main place that this gets used is in deciding how to convert
from IP addresses (layer 3) to MAC addresses (layer 2).  The sending
host does a binary AND of the mask against its own address and that
of the destination it is trying to send to.  If it gets the same
result from these two operations, then both have the same "network"
part of their address -- the destination address appears to be on the
same network segment as the source.  An ARP broadcast of the
destination address should get back an answer showing the MAC address
of the destination, and the packet can be sent there directly.
  But if the result is different, the destination lies on some other
network.  In this case, the sender uses the MAC address obtained for
the *gateway*, which should know how to get packets from this network
to that remote network.

  When you enter a mask that breaks the rules, the results of the AND
operations may match or not, but that doesn't *mean* what it's
supposed to -- a remote address might happen to have those particular
bits in common with the sender, and a local address might not.
  Sending local traffic to the gateway is just inefficient (as long
as the gateway itself (a) exists and (b) is configured properly).
Sending remote traffic to a local address other than the gateway is
not going to work.

David Gillett


On 3 Dec 2001, at 8:32, NICHOLAS BACETTI wrote:

> Just wanted to thank everyone for the great suggestions and comments.
>
> As Tom T. suggested, I went back and changed the subnet mask from
> 10.20.30.40 to 255.255.255.0, and everything is working just fine.
>  Network neighborhood is displaying the other computer on the
> network.  Also, I can now ping by name, as well as, by IP address
> (the latter wasn't a problem, the former was).  This was a good
> point brought up by Dave G.  Finally, as suggested by most, I did
> find the IP and subnet mask numbers, listed with the second
> adapter under the "PPP adapter".
>
> If anyone would care to comment, what was wrong with the
> 10.20.30.40 subnet mask number.  Is this part of a standard
> governing networks?  Again thanks to all...Nick
>
> >>> [log in to unmask] 11/29/01 10:14 AM >>>
> You sound like you know how to set up tcp/ip, but your symptoms also sound
> like an incorrect ip or mask entries.  The first 3 groups of the ip address
> should be the same, and the mask should be 255.255.255.0
> The workgroup under the identification tab needs to be the same on all
> computers, I assume you are not using a domain or a server.
> The reason winipcnf may not display your settings is the first adapter it
> displays is 'PPP adapter'.  Use the drop down arrow to select your hardware
> adapter.  Now your ip and mask settings should appear.  It will only release
> and reset if you are obtaining an ip address automatically.
> Tom Turak
>
> -----Original Message-----
> From: Nicholas Bacetti [mailto:[log in to unmask]]
> Sent: Wednesday, November 28, 2001 3:26 PM
>
> I've set up a two PC network using a hub, both machines are running Win98.
> The problem I'm having is that when I launch network neighborhood, the only
> icons that are displayed are the Entire Network icn, and the host
> computer's icon.  If I further click on Entire Network, no additional icon
> are displayed within Entire Network.  How can I get Network Neighborhood to
> display an icon for the other computer on the network?
>
> FYI, I can ping one computer from the other.  Furthermore, if I go to
> Start>Find>Computer>Name, and enter the name of the other computer, it
> returns a message "computer not found", however if I enter the IP address of
> the other computer, it returns a linked icon to the other computer, which I
> can launch, and which displays those folders and printers that I've set up
> to
>  share.  In fact, I can print from the remote computer.
>
> One last piece of info, when I launch winipconf, the IP addess and subnet
> mask aren't present.  However, they are present under the properies box, in
> the TCP/IP<Network<Control Panel option.  (I manually entered both the IP
> and subnet under  "Specify an IP address").
>
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