[MLUG] 10.* addresses

["McNutt, Justin M." <EMAIL:PROTECTED>]

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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