On Tue, 2003-07-08 at 21:09, Beni Cherniavsky wrote:
> Shaul Karl wrote on 2003-07-08:
>
> > I still don't get something. Quoting section 7 of the IP Sub-Networking
> > Mini-Howto:
> >
> > For the sake of this example, let us assume that you have decided to
> > subnetwork you C class IP network number 192.168.1.0 into 4 subnets
> > (each of 62 usable interface/host IP numbers). However, two of these
> > subnets are being combined into a larger single network, giving three
> > physical networks.
> > These are :-
> >
> > ______________________________________________________________________
> > Network Broadcast Netmask Hosts
> > 192.168.1.0 192.168.1.63 255.255.255.192 62
> > 192.168.1.64 192.168.1.127 255.255.255.192 62
> > 192.168.1.128 192.168.1.255 255.255.255.128 124 (see note)
> > ______________________________________________________________________
> >
> > Note: the reason the last network has only 124 usable network
> > addresses (not 126 as would be expected from the network mask) is that
> > it is really a 'super net' of two subnetworks. Hosts on the other two
> > networks will interpret 192.168.1.192 as the network address of the
> > 'non-existent' subnetwork. Similarly, they will interpret
> > 192.168.1.191 as the broadcast address of the 'non-existent'
> > subnetwork.
> >
> > So, if you use 192.168.1.191 or 192 as host addresses on the third
> > network, then machines on the two smaller networks will not be able to
> > communicate with them.
> >
> > \begin{interruptRequest}
> >
> > How does the 2 smaller networks know that 192.168.1.191 and 192 were
> > initially a broadcast and network addresses? Would they treat any one of
> > 192.168.*.19[12] in the same way?
> >
> > \end{interruptRequest}
> >
> As far as I understand, they assume that sub-networking is uniform:
> they know the full network mask and know it is subnetted with a given
> subnet mask. They assume it doesn't only apply to their own subnet
> but to each subnet of the network. No, they wouldn't treat
> 192.168.*.* in this way, the full network mask seems to be
> 255.255.255.0 so they only assume things about "sibling subnets" i.e.
> 192.168.1.*.
--
Actually, I am not quite sure this is correct. The smaller networks do not do any
assumptions regarding other networks.
The routing decision is done at the router, while the transmitting node is totally
unaware of the subnet mask implemented at the destination network.
If you really want the guts, it depends on the routing protocol implemented on the
router and whether it supports propagating VLSM masks and whether classless routing is
implemented.
As long as routing is done by classless routing protocol, all-zeros and all-ones
subnets become available for use.
As the abbriviation of VLSM applies (Variable Length Subnet Mask), in this case you
have the benefit of not loosing bits for what would look like a broadcast or network
address,
which does not really exists.
In the above axample, with VLSM implemented, the node does the following:
1) The destination address is not in node's network, so it will be transmitted to
default gateway.
2) ARP broadcast to get the MAC address of the default gateway.
3) The node sends the packet to DG
4) DG (the router) performs a lookup in it's routing table and notices that
192.168.1.191 is in 192.168.1.128/25 network which is directly attached to the router.
5) The router queries for the subnet mask associated with the interface and sees
255.255.255.128
5) The packet will be sent through the interface attached to 192.168.1.128/25 (this is
not a broadcast !!!)
6) ARP broadcast to get the destination MAC
7) Packet gets to it's destination.
As you can see, this is a normal unicast address when using VLSM.
References:
1) "Routing TCP/IP. Volume 1" by Jeff Doyle, Cisco Press
2) "Cisco CCNA Exam 640-607 Certification Guide" by Wendell Odom, Cisco Press
Guy
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