[96692] in RedHat Linux List
Re: Private IP address space - why an ISP might _want_ to use
daemon@ATHENA.MIT.EDU (William Stearns)
Wed Oct 28 19:04:11 1998
Date: Wed, 28 Oct 1998 19:07:45 -0500 (EST)
From: William Stearns <wstearns@pobox.com>
To: redhat-list@redhat.com
cc: "Kevin M. Myer" <myer@mail.elanco.k12.pa.us>
In-Reply-To: <Pine.LNX.4.02.9810211438030.1627-100000@marble.elanco.k12.pa.us>
Resent-From: redhat-list@redhat.com
Reply-To: redhat-list@redhat.com
On Wed, 21 Oct 1998, Kevin M. Myer wrote:
> >From RFC 1918:
>
> 3. Private Address Space
>
> 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)
>
> Apparently some people that do routing have never read the RFC's either -
> a traceroute from my building to www.linuxtoday.com shows one fictious IP
> address (sandwiched between two home.net routers) in real address space!
Actually, ISP's commonly use these reserved addresses for
inter-router links. Picture this:
|
|
| Your ISP's
+--Router........................Router--+--->
| ^ ^ ^ To
| | | | The
| Serial | Serial Internet
Office Interface | Interface
LAN |
|
|
Frame relay, ISDN, T1, modem or whatever link
The ISP generally has to give you real IP addresses for the
machines on your lan so people on the Internet can reach those machines.
What about the link between the two routers (marked with the ascii
arrows)? They still (generally) need IP addresses. Nobody should need to
reach them, so the ISP can do any of the following:
- use a block of 4[*] real IP addresses - a very expensive use of a
limited resource.
- use a router like a Cisco that doesn't assign an IP address to the
inside interface, but rather borrows the Ethernet IP address for the
Serial interface (the feature is called "ip unnumbered" - perhaps someday
Linux will be able to have this feature too).
- use a block of 4 reserved IP addresses. Even if the ISP only used the
10.x.y.z block, that still allows for up to 16 million/4=4 million
inter-router links.
An ISP can still put in enough routing information that they can
reach each router for maintenance. By _not_ advertising this information
to the rest of the world, it makes it harder for people outside of the ISP
to try to break into their routers. This is one example of where
reserved (read: unreachable) IP addresses are a benefit.
I've taken a couple of shortcuts with some of the statements, but
the point was to demonstrate that there are reasons why an ISP might want
to use reserved addresses without getting into NAT, static and dynamic
routing, proxy arp, why 4 addresses are required, etc.
Cheers,
- Bill
* 4 addresses are required to connect two machines over a point-to-point
link.
---------------------------------------------------------------------------
Unix _is_ user friendly. It's just very selective about who its friends
are. And sometimes even best friends have fights.
William Stearns (wstearns@pobox.com)
Mason, buildkernel, and named2hosts are at: http://www.pobox.com/~wstearns
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