How to Subnet Class B Addresses.
Subnetting a Class B network is very similar to subnetting a Class C network, with the only difference being that when subnetting a Class B network, you will be focusing on the third octet, whereas in Class C, you will be working on the fourth octet.
Take a look at this:
To subnet Class B, simply use the same subnet numbers for the third octet as in Class C. Just add a zero (0) to the network portion and a 255 to the broadcast section in the fourth octet. Keep in mind that Class B has more possible subnet masks than Class C.
I’ll bring in the cram table again, but this time we’ll apply it to the THIRD octet.
Class B cram table:
VLSM cram table
Class B network address has 16 bits available for host addressing (14 bits for subnetting, 2 bits for host addressing).
Example 1
Let’s look at some examples, using the table above, remember we are working on the THIRD octet 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 octet with the block size of 16.
Network A
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 B
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 C
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
The 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, the 4-bit value gives us /30 which results in mask 255.255.252.0 (just like Class C) so we continue from the next network which is (48+16=64)
WAN 1
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
The next network will also have 4 bits value added to the last network; (64+4=68)
The same four-bit value is used. The next network is:
WAN 2
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 devise a way to make it simple for yourself! I think using the cram table saves you a lot of time from all the equations of all sorts. Let’s apply it to a topology:
Router A:
RA(config)#interface fa0/0
RA(config-if)#ip address 172.16.16.1 255.255.240.0
RA(config-if)#no shutdown
RA(config-if)#exit
RA(config)#interface se0/0/0
RA(config-if)#ip address 172.16.64.1 255.255.252.0
RA(config-if)#no shutdown
RA(config-if)#exit
RA(config)#interface se0/0/1
RA(config-if)#ip address 172.16.68.1 255.255.252.0
RA(config-if)#no shutdown
RA(config-if)#exit
Router B
RB#config t
RB(config)#interface fa0/0
RB(config-if)#ip address 172.16.32.1 255.255.240.0
RB(config-if)#no shutdown
RB(config-if)#exit
RB(config)#interface se0/0/0
RB(config-if)#ip address 172.16.64.2 255.255.252.0
RB(config-if)#no shutdown
RB(config-if)#exit
Router C
RC#config t
RC(config)#interface fa0/0
RC(config-if)#ip address 172.16.48.1 255.255.240.0
RC(config-if)#no shutdown
RC(config-if)#exit
RC(config)#interface se0/0/0
RC(config-if)#ip address 172.16.68.2 255.255.252.0
RC(config-if)#no shutdown
RC(config-if)#exit
Ping from Network RA to RB networks will work.
In conclusion, When subnetting a Class B network, the process involves dividing the third octet into smaller subnets to accommodate the desired number of hosts per subnet. This allows for more efficient use of IP addresses and better organization of network resources.
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