what i understand is as below and correct me if i am wrong
i am using cisco ios Version 12.1(3r)T2,
1) "ip classless" is used to accept classless ip address like 10.132.0.1 255.255.0.0 , if i configure " no ip classless " it will not allow the ip address like this it will only allow classfull ip address ------- but i tried it in the router after configring the " no ip classles " also it accepts the classless ip address for serial interfaces--------why
2) " ip subnet-zero " is used to allow zero subnet ip address for example 10.132.0.1 255.255.0.0,but when i configure " no ip subnet-zero " also it allow me to config the same ip address for the serial inter faces-----why and how ?
Let me start with the more easy one: subnet zero would be 10.0.0.1 255.255.0.0. The address that you used 10.132.0.1 is in subnet 132 not in subnet zero. If you attempt to configure 10.0.0.1 with no subnet-zero configured the router would have given an error message.
To be clear about this subnet zero is a subnet where all the bits in the subnet part of the address are zero. It may help to thing about the address being in three parts: there is the network part of the address (10. in your example), the host part of the address (0.1 in your example) and the subnet part of the address. If all the bits in the subnet part are zero then it is subnet zero.
The answer about ip classless is a bit more complicated. It is not correct to think that no ip classless will not allow you to use a classless address. The router will always allow you to configure an address (with correct subnet mask) and it is difficult to say whether an address is classful or classless. It is not really whether the address is classful or classless but how we determine subnets of the network, how we advertise it, and how we route toward it that is classful or classless.
Some people believe that ip classless or no ip classless restrict what routing protocol can be run but this is not true. IP classless does not restrict what routing protocol can be run. It is very possible to run RIPv1 (a classful protocol) with ip classless and quite possible to run OSPF (a very classless protocol) with no ip classless configured.
It helps me to thing about it in this way: we need to think about how the routing table is built and how the routing table is used to forward packets. It is the responsibility of the routing protocol to build the routing table. And ip classless or no ip classless is not involved in how the routing table is built. IP classless is used in making decisions about how to route packets not in making decisions about how to build the routing table. In particular ip classless determines whether the router will route packets toward an available supernet address.
It might help to have a small example. Assume that you have a routing table like this:
10.51.1.0 Ethernet 0
10.100.1.0 Serial 0
184.108.40.206/16 Ethernet 0
0.0.0.0/0 Serial 0
So the routing table has a couple of subnets of a network, a class B network, and a default route. So if the router gets a packet with destination 10.51.1.26 it is clear that the router will forward through Ethernet 0. If the router gets a packet with destination 220.127.116.11 it is clear that the router will forward through Ethernet 0. If the router gets a packet with destination 18.104.22.168 it is clear that the router will forward through Serial 0. What happens if the router gets a packet with destination 10.200.2.29? At this point we must determine whether the router is configured with ip classless or with no ip classless. If configured with ip classless the router will forward using the default route out Serial 0. If configured with no ip classless the router will drop the packet because with no ip classless the router will not forward toward a supernet address.
This link talks about IP Classless and how it works with the forwarding table. Whether or not you can forward to a supernet or not. Some routing will just be dropped. The No IP classless is ignore sometimes suach as with OSPF. You can configure IP addresses all day they will still be allowed. It is the routing that gets affected.
The link is very good.
As for IP Subnet-zero I haven't actually played with this but 10.132.0.1 255.255.0.0 is a valid subnet. 10.0.0.0 255.255.0.0 would be the subnet that is invalid or subnet zero for the example you gave. Very similar to Host addresses where the Broadcast IP and the Network IP are invalid as host addressess.
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.