Very Computer

What IP addresses are suitable (ie not used in the Internet) to assign
to a network? Part of the network will be connected to a DNS via modem
so simply borrowing some real IP addresses will lead to conflicts.

Any help greatly appreciated

Taken from /etc/hosts :

# According to RFC 1918, you can use the following IP networks for
# private nets which will never be connected to the Internet:
#
#       10.0.0.0        -   10.255.255.255
#       172.16.0.0      -   172.31.255.255
#       192.168.0.0     -   192.168.255.255

Hope this helps,
Peter

        http://info.internet.isi.edu:80/in-notes/rfc/files/rfc1918.txt

you'd see

[from RFC 1918]

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private internets:

     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

This info is also in the default  /etc/hosts on FreeBSD systems.

[from RFC 1918]

                                                   ...Note that (in
   pre-CIDR notation) the first block is nothing but a single class A
   network number, while the second block is a set of 16 contiguous
   class B network numbers, and third block is a set of 256 contiguous
   class C network numbers.

Quote:>      10.0.0.0        -   10.255.255.255  (10/8 prefix)
>      172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
>      192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

Net/Address             Netmask

10.0.0.0                ff000000
172.16.0.0              fff00000
192.168.0.0             ffff0000

The netmask tells the computer how many computers are on it's "local
network".

A netmask of 255.255.255.0 indicates the there are 256 valid address
spaces on the local network.

Of those 256, 254 are usable for workstations, routers, etc, the other
two are reserved for special types of communication at a lower
networking level that don't get sent to the internet or other
networks.

The reserved ip's are xxx.xxx.xxx.0 and xxx.xxx.xxx.255.

iQA/AwUBNkuj5wdgTOglaOBMEQIx4QCfQ8YmO7dG3esmjQgzi9TaceH5XukAoMLV
a6Tktv6eixYjNiTpERxFrmC1
=0JVf
-----END PGP SIGNATURE-----

Quote:>>      10.0.0.0        -   10.255.255.255  (10/8 prefix)
>>      172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
>>      192.168.0.0     -   192.168.255.255 (192.168/16 prefix)
>Net/Address         Netmask
>10.0.0.0            ff000000
>172.16.0.0          fff00000
>192.168.0.0         ffff0000

                                                      Frank Crary
                                                      CU Boulder

Quote:

> Any help greatly appreciated

        10.0.0.0        -   10.255.255.255
        172.16.0.0      -   172.31.255.255
        192.168.0.0     -   192.168.255.255

The RFC can be read at:

  http://www.schooner.com/~loverso/Public/RFC/rfc1597.txt.gz

--
Dave Duchscher                       CIS Network Group
Network Specialist                   Texas A&M University

So, if you have an IP address:

158.152.54.180 netmask 255.255.0.0

Then that means that 158.152.0.0 is the network and on that network, the
host is 158.152.54.180.  Most people are used to seeing that by default
with the old classfull networks, A, B & C.  However, netmasks can be any
length of bitmasks.  For example,

194.217.242.34 netmask 255.255.255.240

Defines a very small network containing the addresses 194.217.242.16 up
to the address 194.217.242.31.  Note that the first address in that
range cannot be used as it is a network address and the last address in
that range is the network broadcast address.  This leaves you with some
14 usable addresses.

Note that netmasks are frequently written as hex strings, because it's
a clearer representation of the bit-level masks than the decimal
version.  eg: 255.255.0.0 could also be expressed as 0xffff0000.
Netmasks are also written as a bit count, eg: 158.152.54.180/16,
represents the same netmask as the previous example.

Urgh.  That was longer than I thought.  That's how I think it works.  If
I'm wrong anywhere, please correct me (I'm not a networking expert).
--
Dom Mitchell -- Palmer & Harvey McLane -- Unix Systems Administrator
``Damn the philosophy, just provide the functionality and let anyone
  with a competing philosophy come up with some better alternative if
  they don't like it.'' -- JKH

rfc1597.txt -- Address Allocation for Private Internets. Y. Rekhter, B. Moskowitz, D. Karrenberg & G. de Groot. March 1994.
(Format: TXT=17430 bytes) (Obsoleted by BCP0005, RFC1918) (Status: INFORMATIONAL)

Quote:> ...but what exactly is a netmask
> and what does it do? Yesterday, I tried to set up a connection between
> a laptop and a desktop machine using lp0 and the parallel printer
> ports. I used 10.0.0.2 for the address of the laptop, which seemed
> to be an appropriate choice. But it did not work. I didn't have
> a chance to put much time into solving the problem, but one of the
> things I noticed was that ifconfig -a reported that the laptop was
> using a netmask of 0xff000000 while the desktop was using a netmask
> of 0xffff0000. Since I'm not sure what a netmask does, I don't
> know if this difference is the problem. Any ideas or suggestions?

The netmask simply says which portion of the IP address is considered
part of the network.  This comes into play in routing,  and subnetting
can significantly reduce the size of routing tables (see Rich Steven's
book,  _TCP/IP Illustrated: Vol. 1_).  You could treat your chosen
address space as 10/8 with 16777214 hosts (2^24 - 2, or the addresses
10.0.0.0 - 10.255.255.255 exclusive of these addresses themselves)
or as 256 different class B networks, 10.0/16 - 10.255/16,  each of
which having 65534 hosts,  or as 65536 class C networks, 10.0.0/24 -
10.255.255/24 each of which has 254 hosts.  There are few other subtle
things,  such as 10.255.255/24 being a bad choice since it looks like
a broadcast address, etc.  But with your choice of 10.0.0.0 you won't
run out of IP addresses for a while.  Just be sure you specify the
same netmask for hosts on the same net.

A couple of basic facts:  a host will not attempt to route packets
if it believes that the address is on a directly-connected network,
it will simply put it on the wire (data/hardware layer);  if a host
does not know a route to a network to which it is not directly
connected,  a network unreachable error results.

Indispensible reference (in addition to any other book by Rich
Stevens!)

Author:        Stevens, W. Richard.
Title:         TCP/IP illustrated / W. Richard Stevens.
Published:     Reading, Mass. : Addison-Wesley Pub. Co.,
                  c1994-c1996.
LC Call No.:   TK5105.55.S74 1994

these addresses are laid down in RFC1918. you you probably read the
full document but this is the relevant part;

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private
internets:

     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

   We will refer to the first block as "24-bit block", the second as
   "20-bit block", and to the third as "16-bit" block. Note that (in
   pre-CIDR notation) the first block is nothing but a single class A
   network number, while the second block is a set of 16 contiguous
   class B network numbers, and third block is a set of 256 contiguous
   class C network numbers.

   An enterprise that decides to use IP addresses out of the address
   space defined in this document can do so without any coordination
   with IANA or an Internet registry. The address space can thus be
used
   by many enterprises. Addresses within this private address space
will
   only be unique within the enterprise, or the set of enterprises
which
   choose to cooperate over this space so they may communicate with
each
   other in their own private internet.

Probaly a typo - and you meant 192.217.242.32 netmask
255.255.255.240, as you surely won't get address in the .16 to .31
range. That would be 192.217.242.15 netmask 255.255.255.240

Quote:>Note that netmasks are frequently written as hex strings, because it's
>a clearer representation of the bit-level masks than the decimal
>version.  eg: 255.255.0.0 could also be expressed as 0xffff0000.
>Netmasks are also written as a bit count, eg: 158.152.54.180/16,
>represents the same netmask as the previous example.

I find the / notation much easier to figure out myself.
--

Regards...