Cisco CCNA: subnetting class B addresses, from start to finish.

In my previous post, I shared how we can subnet a class C address with an example. In this post, I will share how to subnet a class B
address with two examples. Since class B uses two octets for the host bit, I
will show two examples one from each octets. Let’s get going.

Range of class B address: first of all, be reminded that any IP whose values in the first octet fall between 128 and 191, is a class B address.
Example 1: Given
172.16.12.0/19, determine the following:
(i)    The number
of subnets that the given address and subnetmak will produce.
(ii)  The number of
hosts per each of the subnets generated in (i) above
(iii)  Using a table,
list all subnets, first and last valid IPs, and broadcast address
First thing to do is write
the subnetmask from the given slash notation: /19 will be 255.255.224.0 (8 bits
on is 255 and 3 bits on is 224, remember?)
Answers:

(i) To determine the number
of subnets, use 2^X, where X is equal to the number of bits borrowed. This will
give 2^3=8 (the default subnetmask for B is 16 but in this example, we are
given /19, meaning that we borrowed 3.) So our number of subnets will be 8.
Question one answered.

You may also like: Subnetting class A address, from start to finish.

(ii) To determine the number
of host per subnet, use 2^Y-2, where Y equals the number of bits off. This will
give 32-19=13 (IPv4 is a 32-bit address format and in this example, 19 bits are turned on) Now, that will have 13 off, we will substitute that into the
equation, giving us 2^13-2=8,190. We will have 8,190 hosts per subnets (the -2 is for the network and broadcast addresses that we can’t assign to hosts on the networks). 
(iii) To list out the
subnets, we will need a block size. Block size=256-224, which gives us 32. (to
get your block size, simply substitute the value of the subnet bits from 256.
256 is a constant value). Now, that we have a block size of 32, we can list our
subnets in block size of 32, starting from subnet zero (back in the days, IP subnet zero
was not used except you turn it on yourself on the router. Now, IP subnet zero
is turned by default and so, you can use the zero subnet)
Subnet
0.0
32.0
64.0
96.0
128.0
160.0
192.0
224.0
First IP
0.1
32.1
64.1
96.1
128.1
160.1
192.1
224.1
Last IP
31.254
63.254
95.254
127.254
159.254
191.254
223.254
255.254
Broadcast
31.255
63.255
95.255
127.255
159.255
191.255
223.255
255.255

Example 2. Given 172.16.0.0/26, determine the
following:
(i)  Number of unique subnets that the given
address and subnetmask will produce
(ii) Number of hosts per subnet
(iii) List all subnets, valid IP addresses and
their broadcast.
I chose /26 because with it, the subnet bit falls
into the fourth octet, which will be different from the first example where it
fell into the third octet.
First, the subnetmask will be 255.255.255.192

Answers:

(i) Number of subnet will be 2^X where X equals the
number of borrowed bits. This will give us 2^10=1024 (again, the default for
class B is 16 but in this example, we have /29. So when you substitute 16 from
29, you have 10). Answer to question (i) is 1024 subnets.
(ii) How many hosts per subnet? To get this, we use
the formula 2^Y-2 where Y equals the number of off bits. This gives us 2^6-2=62
(again, IPV4 is a 32-bit address and we are given /29. That leaves us with 6
bits off).
(iii) Block size will be 256-192 which will give us
62. With that, we list our subnets in blocks of 62.
First eight subnets
Subnet
0.0
0.64
0.128
0.192
1.0
1.64
1.128
1.192
First IP
0.1
0.65
0.129
0.193
1.1
1.65
1.129
1.193
Last IP
0.62
0.126
0.190
0.254
1.62
1.126
1.190
1.254
Broadcast
0.63
0.127
0.191
0.255
1.63
1.127
1.191
1.255

Last eight subnets

Subnet
254.0
254.64
254.128
254.192
255.0
255.64
255.128
255.192
First IP
254.1
254.65
254.129
254.193
255.1
255.65
255.129
255.193
Last IP
254.62
254.126
254.190
254.254
255.62
255.126
255.190
255.254
Broadcast
254.63
254.127
254.191
254.255
255.63
255.127
255.191
255.255

If you have any question on this or other issues, please use the comment box. Thanks.

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Timigate

Ashioma Michael, a BSc (Computer Science)., MTCNA, CCNA, and CCNP holder with many years of industry-proven experience in network design, implementation and optimization. He has tutored and guided many professionals towards obtaining their Cisco certifications. Mike works as a senior network engineer with one of the leading internet service providers in West Africa.

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