Subnetting Class B Addresses.
Subnetting Class B network is much more similar to subnetting Class C, the only difference is that when subnetting class B, you will be working on the third octect; while Class C, you will work on the fouth octect.
Look at this:
To enable you subnet Class B, use the same subnet numbers for the third octect just as in Class C. All you need to do is just to add zero (0) to the network portion and a 255 to the broadcast section in the fourth octect. Remember we have more possible subnet mask in Class Bthan Class C.
I will bring in the cram table once more, only this time we are applying it on the THIRD octect;
Class B cram table:
Class B network address has 16 bits available for host addressing (14 bits for subnetting, 2 bits for host addressing).
Let’s look at some examples, using the table above, remember we are working on the THIRD octect of Class B. Given network address:172.16.0.0 /20
From the above network IP address, the mask will be 255.255.240.0 which means we are using the bit value or block size of 16.
We are going to subnet it to three different networks with equal host IP addresses; remember we are working on the THIRD octect with the block size of 16.
Network address: 172.16.16.0
First Host address: 172.16.16.1
Last host address: 172.16.31.254
Broadcast address: 172.16.31.255
What we did above is to add the bit value or size (16+16=32) to obtain the next network address which is 172.16.32.0
Network address: 172.16.32.0
First Host address: 172.16.32.1
Last host address: 172.16.47.254
Broadcast address: 172.16.47.255
We carried out the same addition here to get the next network address (32+16=48)
Network address : 172.16.48.0
First Host address : 172.16.48.1
Last host address: 172.16.63.254
Broadcast address: 172.16.63.255
Same addition before for the next network.
For the WAN (serial links) We need only 4 bits value or block size here due to the number of network and hosts involved so as not to waste much address space. looking at the cram table, 4 bit value gives us /30 which results to mask 255.255.252.0 (just like Class C) so we continue from the next network which is (48+16=64)
Connection from Router A to Router B
Network address: 172.16.64.0
Network A to B address: 172.16.64.1 255.255.252.0
Network B to A address: 172.16.64.2 255.255.252.0
Next network will also have 4 bits value added to the last network; (64+4=68)
Same four bit value is used. The next network is:
Connections from Router A to Router C
Network address: 172.16.68.0
Network A to C address: 172.16.68.1 255.255.252.0
Network C to A address: 172.16.68.2 255.255.252.0
There are different ways to subnet; you have to device a way to make it simple for yourself! I think by using the cram table saves you a lot of time from all the equation of all sort. Lets apply it to a topology:
RA(config-if)#ip address 172.16.16.1 255.255.240.0
RA(config-if)#ip address 172.16.64.1 255.255.252.0
RA(config-if)#ip address 172.16.68.1 255.255.252.0
RB(config-if)#ip address 172.16.32.1 255.255.240.0
RB(config-if)#ip address 172.16.64.2 255.255.252.0
RC(config-if)#ip address 172.16.48.1 255.255.240.0
RC(config-if)#ip address 172.16.68.2 255.255.252.0
Ping from Network RA to RB networks will work.