Subnetting Class B

In this article we will understand IPv4 Class B subnetting. Let's begin by understanding basic definitions.
 
Definition of IP Address:  An IP address is a 32 bit number that logically defines a host. IP address assigned to a LAN card or a device interface can change based on network admin's requirement.

Definition of Subnet Mask: Subnet Mask is a 32 bit value that defines which portion of an IP address is network and which portion is host.

Definition of Subnetting and its requirement: Subnetting is extending the network portion and reducing the host portion. Advantage of Subnetting is that helps a network admin to efficiently use the network IPs.

Before understanding Class B subnetting in detail, it is advised to have clear understanding of  Subnetting Class C

Let's take example of a Class B Network 172.16.0.1/16. We will subnet this network from 172.16.0.0/16 till 172.16.0.254/30. Kindly note that default subnet mask of  Class B is /16 (or 255.255.0.0).

Kindly note that subnet mask 255.255.0.0 can be written as /16.   We can write /16 in binary as  11111111.11111111. 00000000.00000000. Here, number of 1s is 16 and we use this value to denote the subnet mask as /16. In Class B network shown above, we will have 16 Network bits (1s) and 16 host bits (0s) and using subnetting we will extend  the network portion from  172.16.0.0/16 till 172.16.0.0/30. 

 

Subnetting Class B (172.16.0.0/17)

Let's begin subnetting by borrowing 1 bit (Most significant bit) from host portion and use it in the network portion.  The subnet mask will become /17 (previous value was /16).

Also note that the last 2 octets (3^rd and 4^th octets) of network 172.16.0.0 can be written as 00000000.00000000

While we change subnet mask from /16 to /17, the binary value of  3^rd  octet of subnet mask can be written as 10000000 while the binary value of 4^th octet will remain 00000000. The Decimal value of subnet mask will become 255.255.128.0, because the decimal value of  10000000 in 3^rd Octet is 128.

Let assume the number of bits we have made turned '1' or borrowed = N( ex. Network bit),  so here N= 1

Let assume the number of bits left zero(remaining) = H (ex. Host bit), so here H = 15.

The binary format of 3^rd  and 4^th octet will be : 10000000.00000000 (7 bits of 3^rd Octet are 0 and all 8 bits of 4^th octet are 0. So 7+8 = 15)

Important Formulas to calculate number of subnets and hosts per subnet

• Number of subnets   =  2^N
  
• Number of hosts per subnet =  (2^H)   - 2
• Block size of each subnet =  256 - Decimal value of  new mask.

 

Subnetting Class B (172.16.0.0/17)

Based on above rules let's calculate the values for 172.16.0.0/17

• Number of subnets (for /25 mask)  =  2^N   

                 N = 1, so 2¹   =  2 ,  So Number of subnets (for /25 mask) = 2

                 We will also discuss about range/value/block size of these subnets in this article shortly.

• Number of hosts per subnet =  (2^H)   - 2  

                  H is 7, so 2¹⁵ - 2 
                  Hosts per subnet = 32768 - 2 = 32766.

       

• Block size of each subnet =  256 - 128  = 128

For /17 mask, we will get 2 subnets, each with 32766 hosts. Below is list of 2 subnets:

•     1st  Subnet  = 172.16.0.0/17
•     2nd  Subnet = 172.16.128.0/17

Below is list of range of IP address of these subnets.

•  For 1st  Subnet, 1st valid IP is 172.16.0.1/17, last valid IP is 172.16.127.254/17, and broadcast     IP is 172.16.127.127/17
•  For 2nd  Subnet, 1st valid IP is 172.16.128.1/17, last valid IP is 172.16.255.254/17, and broadcast IP is 172.16.255.255/17

 

Subnetting Class B (172.16.0.0/23)

With mask /23, subnet mask will be 255.255.254.0.  Binary value of 3^rd  octet is 11111110 and 4^th Octet is 00000000. Using this let’s proceed further.

• Number of subnets (for /23 mask)  =  2^N      

              N = 7,  (2 bits are borrowed from host portion), so 2⁷  =  128
              Number of subnets (for /23 mask) = 128

• Number of hosts per subnet =  (2^H)   - 2     

     H is 9, (1 zero from 3^rd octet and 8 zero bits from 4^th octet are available for host portion), so 2⁹  - 2.
             Hosts per subnet = (512 -2) = 510.

• Block size of each subnet =  256 - 254  = 2

For /23 mask, we will get 128 subnets, each with 510 hosts, each subnet has block size of 2. Below is list of all 128 subnets.

• 1^st  Subnet  = 172.16.0.0/23
• 2^nd  Subnet = 172.16.2.0/23
• 3^rd  Subnet = 172.16.4.0/23
• 4^th  Subnet = 172.16.6.0/23
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• 127^th  Subnet=172.16.252.0/23
• 128^th  Subnet=172.16.254.0/23

 Below is list of range of IP address of these subnets.:

•  For 1^st  Subnet, 1st valid IP is 172.16.0.1/23, last valid IP is 172.16.1.254/23, and broadcast IP is 172.16.1.255/23
  
•  For 2^nd  Subnet, 1st valid IP is 172.16.2.1/25, last valid IP is 172.16.3.254/23, and  broadcast IP is 172.16.3.255/23
•  For 3^rd  Subnet, 1st valid IP is 172.16.4.1/23, last valid IP is 172.16.5.254/23, and broadcast IP is 172.16.5.255/23
•  For 4^th  Subnet, 1st valid IP is 172.16.6.1/23, last valid IP is 172.16.7.254/23, and  broadcast IP is 172.16.7.255/23
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• For 127^th  Subnet, 1st valid IP is 172.16.252.1/23, last valid IP is 172.16.253.254/23, and  broadcast IP is 172.16.253.255/23
• For 128^th  Subnet, 1st valid IP is 172.16.254.1/23, last valid IP is 172.16.255.254/23, and  broadcast IP is 172.16.255.255/23 

Subnetting Class B (172.16.0.0/24)

With mask /24, subnet mask will be 255.255.255.0.  Binary value of  3^rd  octet is 11111111 and 4^th Octet is 00000000. Using this let’s proceed further.

• Number of subnets (for /24 mask)  =  2^N    

             N = 8,  (all 8 bits of 3^rd Octet are borrowed for network portion), so 2⁸ = 256
             Number of subnets (for /24 mask) = 256

• Number of hosts per subnet =  (2^H)  - 2     

            H is 8, (all 8 bits of 3^rd octet are borrowed as 1 and 8 bits from 4^th octet are zero) so  2⁸ - 2. 
            Hosts per subnet = (256 -2) = 254.

• Block size of each subnet =  256 – 255(decimal value of 3^rd octet with all 1s)  = 1

For /24 mask, we will get 256 subnets, each with 254 hosts, each subnet has block size of 1. Below is list of  all 256 subnets:

• 1^st  Subnet  = 172.16.0.0/24
• 2^nd  Subnet = 172.16.1.0/24
• 3^rd  Subnet = 172.16.2.0/24
• 4^th  Subnet = 172.16.3.0/24
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• 255^th  Subnet=172.16.254.0/24
• 256^th  Subnet=172.16.255.0/24 
  

  Below is list of range of IP address of these 256 subnets:

• For 1^st  Subnet, 1st valid IP is 172.16.0.1/24, last valid IP is 172.16.0.254/24, and broadcast IP is 172.16.0.255/24
• For 2^nd  Subnet, 1st valid IP is 172.16.1.1/24, last valid IP is 172.16.1.254/24, and  broadcast IP is 172.16.1.255/24
• For 3^rd  Subnet, 1st valid IP is 172.16.2.1/24, last valid IP is 172.16.2.254/24, and broadcast IP is 172.16.2.255/24
• For 4^th  Subnet, 1st valid IP is 172.16.3.1/24, last valid IP is 172.16.3.254/24, and  broadcast IP is 172.16.3.255/24
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• For 255^th  Subnet, 1st valid IP is 172.16.254.1/24, last valid IP is 172.16.254.254/24, and  broadcast IP is 172.16.254.255/24
• For 256^th  Subnet, 1st valid IP is 172.16.255.1/24, last valid IP is 172.16.255.254/24, and  broadcast IP is 172.16.255.255/24

Subnetting Class B (172.16.0.0/26)

With mask /26, subnet mask will be 255.255.255.192.  Binary value of  3^rd  octet is 11111111 and 4^th Octet is 11000000. Using this let’s proceed further.

• Number of subnets (for /26 mask)  =  2^N    

             N = 10,   (2 bits are borrowed from host portion), so 2¹⁰  =  1024
             Number of subnets (for /26 mask) = 1024

• Number of hosts per subnet =  (2^H) - 2     

            H is 6, (6 bits of from 4^th octet are left zero), so 2⁶ - 2. 
            Hosts per subnet = (64 -2) = 62.

To find Block Size, we need to be careful in calculating the block size because block size for 172.16.0.0/26 will be calculated from 3^rd and 4^th octets both.

• Block size from 3^rd Octet is 1 and Block size from 4^th Octet is 64.  I know you have got confused. But once you look the example below, things will be clearer.
• Block size of each subnet(3^rd Octet) =  256 – 255 =1        (decimal value of 3rd  octet with all 1s). 
  
• Block size of each subnet (4^th Octet) =  256 – 192  = 64   (decimal value of 4^th octet with  two 1s).
  

For /26 mask, we will get 1024 subnets, each with 62 hosts, each subnet has block size of 64. Below is list of  all 1024 subnets:

• 1^st  Subnet  = 172.16.0.0/26
• 2^nd  Subnet = 172.16.0.64/26
• 3^rd  Subnet = 172.16.0.128/26
• 4^th  Subnet = 172.16.0.192/26
• 5^th   Subnet  = 172.16.1.0/26
• 6^th   Subnet = 172.16.1.64/26
• 7^th   Subnet = 172.16.1.128/26
• 8^th   Subnet = 172.16.1.192/26
• 9^th   Subnet  = 172.16.2.0/26
• ^10th   Subnet = 172.16.2.64/26
• ^11th   Subnet = 172.16.2.128/26
• 12^th  Subnet = 172.16.2.192/26
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• 1021^st   Subnet  = 172.16.255.0/26
• 1022^nd   Subnet = 172.16.255.64/26
• 1023^rd   Subnet = 172.16.255.128/26
• 102^4th  Subnet = 172.16.255.192/26

 
Below is list of range of IP address of these 1024 subnets.:

• For 1^st Subnet, 1st valid IP is 172.16.0.1/26, last valid IP is 172.16.0.62/26, and broadcast IP is 172.16.0.63/26
• For 2^nd  Subnet, 1st valid IP is 172.16.0.65/26, last valid IP is 172.16.0.126/26, and  broadcast IP is 172.16.0.127/26
• For 3^rd  Subnet, 1st valid IP is 172.16.0.129/26, last valid IP is 172.16.0.190/26, and broadcast IP is 172.16.0.191/26
• For 4^th  Subnet, 1st valid IP is 172.16.0.193/26, last valid IP is 172.16.0.254/26, and  broadcast IP is 172.16.0.255/26
• For 5^th  Subnet, 1st valid IP is 172.16.1.1/26, last valid IP is 172.16.1.62/26, and broadcast IP is 172.16.1.63/26
• For 6^th   Subnet, 1st valid IP is 172.16.1.65/26, last valid IP is 172.16.1.126/26, and  broadcast IP is 172.16.1.127/26
• For ^7th   Subnet, 1st valid IP is 172.16.1.129/26, last valid IP is 172.16.1.190/26, and broadcast IP is 172.16.1.191/26
• For 8^th   Subnet, 1st valid IP is 172.16.1.193/26, last valid IP is 172.16.1.254/26, and  broadcast IP is 172.16.1.255/26
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• For 1021^st   Subnet, 1st valid IP is 172.16.255.0/26, last valid IP is 172.16.255.62/26, and  broadcast IP is 172.16.255.63/26
• For 1022^nd  Subnet, 1st valid IP is 172.16.255.65/26, last valid IP is 172.16.255.126/26, and  broadcast IP is 172.16.255.127/26
• For 1023^rd   Subnet, 1st valid IP is 172.16.255.129/26, last valid IP is 172.16.255.190/26, and  broadcast IP is 172.16.255.191/26
• For 1024^th  Subnet, 1st valid IP is 172.16.255.193/26, last valid IP is 172.16.255.254/26, and  broadcast IP is 172.16.255.255/26

 
I hope you have understood the concept of Class B subnetting. Kindly try the subnetting for 172.16.0.0/28.  Kindly share this article with your friends too. 

 

 

For Further Reading

 Subnetting Class C