RE: [MLUG] 10.* addresses

["Brent Deterding" <EMAIL:PROTECTED>]

Is 192.168.0.0/16 a Class B address? 

It is not - although commonly it would be referred to as that. Strictly
speaking a "Class A" has the first octet as 1-127 inclusive, a B 128-191
inclusive, and a C 192-254 inclusive. 

-- Brent

-----Original Message-----
From: EMAIL:PROTECTED
[mailto:EMAIL:PROTECTED]On Behalf Of McNutt, Justin M.
Sent: Thursday, August 01, 2002 11:01 AM
To: EMAIL:PROTECTED
Subject: [MLUG] 10.* addresses


Incidentally, a word about the private IP address space:

There are several blocks of IP's that are considered "private".  They are:

10.0.0.0/8 (one class A)
172.16.0.0/12 (16 class B's)
192.168.0.0/16 (256 class C's)

127.0.0.0/8 (loopback)

169.254.0.0/16 (related to DHCP - one class B)

239.0.0.0/8 (multicast - one class A)

Contrary to popular belief, these are not "non-routable" or "LAN-only"
addresses.  They are private space, which means that they will never be
assigned to hosts that are globally addressable.  "Global" vs. "private" is
the distinction to be made here.

One could make an entire enterprise with 700 subnets, routers, switches,
servers, etc. using only the 10.0.0.0/8 address space.  This works fine,
even though you cross many router hops to get from one subnet to another.
The only catch is that you must be NAT'd/masq'd/PAT'd before you talk to
anything on the commodity Internet.

Put another way:  From a router's point of view, there is nothing at all
special about these addresses.  The only thing special about them at all is
the fact that ISP's filter them out - manually, I might add - at the
customer edge (or perhaps, at their boundaries with other ISP's).

So the routers by default treat these networks are regular old IP networks.
Only filters or policies make them act differently.

--J

P.S.  Exception to everything I said:  *Some* routers will send packets
destined for 127.0.0.0/8 to their default gateways (route them normally).
Other routers will treat them as destined for the local router's management
interface (as if the packet were destined to the router's own loopback
interface).  Either way, this is totally non-legit traffic and should be
filtered both inbound and outbound on any routing interface.

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