Base Address

Bolton07

'If two networks have the same base address and subnet mask, a router views them as identical'


What does the term Base address mean in the context above?

JamesFigueroa

I believe it means if you have two addresses that are in the same class(base) and same subnet mask, the router views them as identical(summarization when dynamic routing is being used)

ex:
192.168.1.56
255.255.255.0

and

192.168.1.83
255.255.255.0

will show up in dynamic routing as
192.168.1.0

Identical in the sense that they will be summerized into the classful representation of the network is my guess. Can you give more info on the rest of the context?

Bolton07

here is the rest of all the information that follows-

If two networks have the same base address and subnet mask, a router views them as identical. It considers a summarized network, as well as an individual network that is part of that summary, as different networks.

The summarized network 192.168.0.0/22 and the individual network 192.168.1.0 /24 are different entries, even though the summarization includes the individual network. When this situation occurs, both networks are placed in the routing table. The decision of which route to use falls to the entry with the closest, or longest, prefix match.

As an example, a router receives a packet with a destination IP address of 172.16.0.10. Three possible routes match this packet: 172.16.0.0/12, 172.16.0.0/18, and 172.16.0.0/26. Of the three routes, 172.16.0.0/26 has the longest match. For any of these routes to be considered a match, there must be at least the number of matching bits indicated by the subnet mask of the route.

JamesFigueroa

This seems to be explaining the process of a router consulting its routing table to find the best route using parent and child routes.

Basically, when you turn on dynamic routing, a router will "summarize" the network or subnet into its classful representation(A,B,C) with the default subnet mask. This is the "parent" route and is usually the first address you see when looking at a routing table group.

Underneath the "parent" route, it will list the subnets for that network. What your article is describing is the router looking at the routing information on a packet and matching it up first against the parent route and if there are child routes it will attempt to match up against them to find the best route.

For example:

If a router receives a packet destined for the 172.16.0.10 network, it first will look at its routing table. Lets say it looks like this:

172.16.0.0/16 is variably subnetted, 4 subnets
172.16.0.0/12 is directly connected, FastEthernet0/0
172.16.0.0/18 is directly connected, FastEthernet0/1
172.16.0.0/26 is directly connected, FastEthernet1/0
172.17.0.0/16 is directly connected, FastEthernet1/1
192.168.1.0/24 is directly connected, Serial0/1

The router will break up the IP packet destination into binary:
172.16.0.10 = 10100011.00010000.00000000.00001010

The router will then match the binary of that IP packet against its "parent" routing table first to find the best match. In this example, 172.16.0.0 and 192.168.1.0 are the parent routes:

172.16.0.0= 10100011.00010000.00000000.00000000
192.168.1.0 = 11000000.10101000.00000001.00000000

Since there are no more parent routes in the table, the router will see that the 172.16.0.0 parent route has the most bit in common with the IP packet in question(172.16.0.10). The router then realizes that this parent route also has "child routes". It will do the same process again, this time against the child routes to find out if there is a child route that has more bits in common with the IP packet in question than the parent route. Note that the router takes the subnet mask into consideration when determining the number of bits to use for comparison against the IP packet:

172.16.0.0/12 = 10100011.00010000.00000000.00000000(since this has a /12 mask, only the first 12 bits will be taken into account for comparison.)

-The router found 12 bits in common with the IP packet in question. It moves onto the next child route

172.16.0.0/18 = 10100011.00010000.00000000.00000000(since this has a /18 mask, only the first 18 bits will be taken into account for comparison.)

-The router found 18 bits in common with the IP packet in question. Since this has more bits in common than the previous route, right now this is winning. It moves onto the next child route

172.16.0.0/26 = 10100011.00010000.00000000.00000000(since this has a /26 mask, only the first 26 bits will be taken into account for comparison.)

-The router found 26 bits in common with the IP packet in question. Since this has more bits in common than the previous route, right now this is winning. It moves onto the next child route

172.17.0.0/16 = 10100011.00010001.00000000.00000000(since this has a /16 mask, only the first 16 bits will be taken into account for comparison.)

- The router only found 15 bits to match so this route is obviously no good.


Now that the router determined the 172.16.0.0/26 to have the most bits in common with the IP route in question(172.16.0.10), the router will proceed to process the packet through (in this case) the interface.

I hope this helps you bud. Keep in mind a big note, the IOS needs to be functioning in a classless routing behavior. If the IOS is not functioning in classless routing behavior, the router will not continue to investigate the child routes to find the best match.

Bolton07

Thanks alot. That was very helpfull.

By the way I thought that dynamic routing protocols were the only things that could be classless.

I did not know that the IOS could be classless too. Also how would you configure an IOS to be Classless?

JamesFigueroa

Im glad to hear it was helpful!

With the newer IOS', classless behavior is enabled by default(a good thing). You should be able to check on the running config or version if it is enabled.

Anyway, the command is:

router(config)#ip classless

and of course to disable its:

router(config)#no ip classless