Private in-house domain

Python python at venix.com
Thu May 17 20:06:25 EDT 2007


On Thu, 2007-05-17 at 16:10 -0400, Tech Writer wrote:
> Reading some of the replies, I wanted to clarify my initial problem just a 
> little...  I realize this isn't a setup anyone would put together in the 
> "real" world, but it seemed to fit the environment I needed.  Let me try to 
> explain just a little bit...
> 
> My task (as a technical writer) was to take a DNS and mailserver sample, and 
> implement it on some test machines.  If this were in a closed environment, 
> I'd just pick any address range, and set it up without any concern for the 
> machines around me.  But, when I'm done, I need to temporarily set this up 
> in a larger lab, used for training, at the customer site.
> 
> The problem is that the customer's lab has a "real" DNS server, IP 
> addresses, etc.  The machines I will be allocated are currently in the lab, 
> and have pre-defined addresses in the 192.168.1.0/24 range.  I can not mess 
> around with these addresses, nor can I allocate new ones, etc.  However, I 
> DO need to set up a DNS server.  So, since I can't control the addresses 
> that are already there, the simplest thing seemed to be to just grab my own 
> set of addresses (that aren't being used in that lab) and have my DNS server 
> control this alternate set of addresses.
> 
> So, I gave my DNS server, and its client addresses in the 10.25.1.0/24 
> range.  

This is where you lose people like me.  Why not simply configure your
test DNS to use your existing addresses or change your IP addresses to
match the lab?

I think Paul pointed out that DNS is simply a scheme for providing a
hierarchy of names that will map to IP addresses.  So keep your
connectivity working by sticking with your current numbers.  Configure
your name server with the current numbers.  Then change /etc/resolv.conf
(or use the appropriate configuration application) on a computer to use
your test name server.

> That was fine, except that they couldn't get out to the rest of the 
> world.  When I added the alias for eth0 (so it could also use 192.168.1.30) 
> and set the gateway to 192.168.1.1, then everything seemed to work fine.
> 
> I did this in my own home network environment.  But when it's working, I 
> will be duplicating it on a couple of machines in the training lab.  If 
> someone can think of a better way that this could have been implemented, I'm 
> always open to suggestions.

I started to recommend the first chapter of "Foundations of Python
Network Programming" which is in our library, but discovered it
completely glosses over netmasks and subnets.  None of the other books
seem to be any better.  I'll attempt a quick explanation.

IP addresses are normally assigned in conjunction with a netmask.  The
address identifies a computer.  The netmask provides the information
need to distinguish local and remote addresses: a count of bits.
255.255.255.0 means 24 bits.  (255 is the value of 8 bits all set to
ones)  So 255.255.255.0 has 3 sets of 8 bits == 24 bits.
255.255.255.128 == 25 bits.  255.0.255.0 is illegal!  We need to specify
a continuous set of one bits counting from the left.  It is a very
clumsy notational system.  You'll often see something like
        192.168.0.40/24
which is much simpler to read and understand.

All addresses that match bit-for-bit for the number of bits specified in
the net mask, get delivered by your local network.  Any address that
fails to match must be given to a router for delivery.  Clearly you need
a router with a local address for this scheme to work.  You need your
local network to deliver the message to the router.

So an address of 10.25.1.40/24 should be configured with a gateway of
10.25.1.X where X is a number from 1 to 254 that has been assigned to a
local router.  The router will be configured to handle all addresses
that do NOT begin with 10.25.1.  Usually that's simply a matter of
sending the messages off to the ISP.

The lowest and highest addresses in your local address range are
reserved.  The lowest identifies the network,  the highest is used for
local broadcasts.

I hope that was somewhat coherent.

> 
> Peg 
> 
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-- 
Lloyd Kvam
Venix Corp



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