I do not really understand your question. But let me explain a few things that seem related and perhaps that will provide an answer to your question.
First let me clarify the 3 lines of the routing table entry
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.16.0/30 is directly connected, Serial0/1/0
L 172.16.16.1/32 is directly connected, Serial0/1/0
- The first line of output identifies the major classful network ID. In this case it is the class B network 172.16.0.0. (note that this entry is not associated with any particular interface) And this parent entry shows that the routing table contains 2 subnets and 2 subnet masks for networks that belong in this major network.
- The second line contains a child entry with more detail. This is for a particular subnet of the major network 172.16.16.0. (and the entry tells us that we can get to that particular subnet by using interface Ser0/1/0). And the entry tells us that the subnet mask for this subnet is /30.
- The third line contains a child of the child with even more detail. This is for a host address within that subnet 172.16.16.1 (and the entry tells us that we can get to that particular address by using interface Ser0/1/0) And the entry tells us that the subnet mask for this entry is /32.
Many of us learned about subnetting using a simple rule about the classes of networks and the networks contained in each class:
class A networks 1 through 126
class B networks 128 through 191
class C networks 192 through 223
multicast network 224 and above
In that rule it sort of looks like the only octet that matters is the first octet. But if we look more closely at the addresses we will need to recognize that part of the IP address is the network address, part of the IP address is the subnet address, and part of the IP address is the host address.
If we look at the first octet of the address we find that will always be part of the network address.
If we look at the second octet of the address we find that it might be part of a network address, but it might be part of a subnet address (and might even be part of a host address)
If we look at the third octet of the address we find that it might be part of a network address, that it might be part of a subnet address, or that it might be part of a host address.
If we look at the fourth octet of the address it will never be part of a network address, but it might be part of a subnet address, and it will be part of a host address.
I hope that this helps you to figure out what comes in the second octet and so on. If you still have questions please respond by clarifying what you want to know.
HTH
Rick
