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Re: [vox-tech] dsl ideas
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Re: [vox-tech] dsl ideas



On Mon, 4 Dec 2000, Jay wrote:

> 
> ----- Original Message -----
> From: <jdnewmil@dcn.davis.ca.us>
> To: <vox-tech@franz.mother.com>
> Sent: Sunday, December 03, 2000 11:52 PM
> Subject: Re: [vox-tech] dsl ideas
> 
> 
> > On Sun, 3 Dec 2000, Jay wrote:
> >
> > > I thought the idea behind 5 ips is:
> >
> > This is "an" idea... one of many possible...
> >
> > >
> > > 1 ip firewall/NAT/Router/Gateway (3 NICs, 1 for external, 1 for your
> internal, 1
> > > for your DMZ
> > >
> > > DMZ
> >
> > Not sure what you mean by lumping "DMZ" with individual ip numbers... a
> > DMZ is a network... a group of ip numbers.
> >
> > > 2 ips DNS
> 
> I mixed my metaphors, I was talking about the NICs on the firewall, 1 for
> internet, 1 for DMZ, 1 for your internal network.  1 ip for your firewall to
> communicate to the external world, the other 4 are the DMZ (which communicate
> thru the firewall).
> 
> >
> > Seems better to arrange for someone else to handle your backup DNS
> >
> 
> I suppose, but don't you have to pay for that then?

I don't run dns... I use a free dns server.  But if I did, I would
(a) run dns on a server with other services to make most use of the ip,
and (b) try to find someone else doing dns and barter ("I'll back you up
if you back me up").

> 
> > > 2 - pop/imap, HTTP, SMPT, and whatever else external services
> >
> > 5 ips arises because a 29 bit netmask leaves 3 bits for your network,
> > which would be 8 addresses except that the network and broadcast addresses
> > are usually reserved, and the isp needs to keep an address to configure
> > the upstream router with.  This is about the smallest practical network
> > size.
> 
> Makes sense.  Why isn't the netmask 36 bits? Isn't it 4 bytes long?

The whole mask is 32 bits (4 bytes) long. There is no parity involved. :)

The portion filled with ones can vary, and 29 "1" bits is often referred
to as a 29 bit netmask.

>  How do I
> figure out what the range of IPs I have if I know my netmask and I know the
> first IP.

Use a netmask calculator to get comfortable with it, or just to avoid
bitbanging... http://www.telusplanet.net/public/sparkman/netcalc.htm (uses
JavaScript)

> i.e. if I'm using netmask of 255.255.255.248 and my IP = 192.168.1.0

Strictly speaking, if that is your netmask then your ip should not be that
one:

      192.168.1.0     = 11000000.10101000.00000001.00000000  (ip?)
 and  255.255.255.248 = 11111111.11111111.11111111.11111000  (mask)
 ----------------------------------------------------------
      192.168.1.0     = 11000000.10101000.00000001.00000000  (network)

A host ip should not be the same as a network number... that is, the host
bits (the last three in this case) should not be all zeros for a host ip
address. (It is technically possible that a host address could be the same
as a network address, but the "traditional" arrangement doesn't permit
it.)

If you had a different netmask...

      192.168.1.0     = 11000000.10101000.00000001.00000000  (ip?)
 and  255.255.254.0   = 11111111.11111111.11111110.00000000  (mask)
 ----------------------------------------------------------
      192.168.0.0     = 11000000.10101000.00000000.00000000  (network)

then that address would be "legal", since it would not be the same as the
network number.

The other reserved number is the broadcast address:

      192.168.1.1     = 11000000.10101000.00000001.00000001  (better ip)
 or   0.0.0.7         = 00000000.00000000.00000000.00000111  (not mask)
 ----------------------------------------------------------
      192.168.1.7     = 11000000.10101000.00000001.00000111  (broadcast)

The host addresses (more accurately the "interface addresses" but the term
"host" is traditional) fill in between 192.168.1.0 and 192.168.1.7.  Only
your ISP can tell you which ip number they chose from among the host ip
numbers for your gateway, but it is typically the first one after the
network number or the last one before the broadcast address.

Thus, for an "n" bit netmask, the number of usable ip addresses for your
network would be 2^(32-n)-3.

---------------------------------------------------------------------------
Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil@dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live Go...
Work:<JeffN@endecon.com>              Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...2k
---------------------------------------------------------------------------



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