IP Subnetting-The Easy Way

Is network subnetting driving you crazy ? Does binary math gives you headaches ? well..fear not, since the time i have delved into networks, i have gone through all the oohs and aaahs and the NAH’s of the logical crescendo , and I present you the easiest way to do subnetting. Generally,3 questions are being addressed when subnetting an IP-

1. No of subnets
2. No of valid hosts and
3. Host range/block size

We will tackle them one by one. First you need to know about IP addresses and their classes.

Class A includes 0-127 where 0 and 127 are reserved, the default subnet mask for this class is /8 .

Class B includes 128-191 in their first octet, and the default subnet mask for this class is /16

Class C deals with 192-224 in their first octet and the default subnet mask for this class is /24

Also, also, understand a simple concept, subnet masks lying between 8- 15 are A class masks, from 16- 23 are B class and 24-32 are C class masks. Furthermore, a subnet mask can be expressed as this where (N)etwork value and (H)ost values-

Class A : 255.0.0.0   = NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH   
Class B : 255.255.0.0 = NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH  
Class C : 255.255.0.0 = NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH 
Class mask value          8        16        24        32

Now once you see it, lets tackle some real life questions. Lets find the no of subnets and valid hosts for192.168.10.10/18

See this ? its a C class IP address having a mask of B class (as the mask lies between 16-24) now, in order to find the number of subnets, use the following formulae - 

2 ^ (What mask you have been provided – default mask of the IP address given)

putting the values here..

2^(18-16) –>  2^(2) –>  4 subnets

simple : )

Now for calculating the no.of hosts, use the below formulae -

2^(32- what mask you have been provided) –2

Putting values here..

2^(32-18)-2 -> 2^(14)-2 –> 16384-2 –> 16382 hosts

piece of cake ..

now to find the block size, see the provided mask lies between which next default mask value , which in this case is 24 (as 18 is greater than 16 and less than 24) . So ..

Subtract the provided mask with the class mask value which is greater than it.

2^(Next class mask value – provided mask)

which on putting values will be

2^(24-18) –> 2^6 –> 64

So, the block size will be of 64 . So, the IP addresses will be divided into 4 subnets (which we already calculated above) above as -

192.168.0.0 - 192.168.63.255

192.168.64.0 – 192.168.127.255

192.168.128.0 - 192.168.191.255

192.168.192.0 – 192.168.255.255

And the best part, its applicable to all classes : 

Hack BSNL Broadband For Speed

If you are a BSNL broadband user, chances are that you are facing frequent DNS issues. Their DNS servers are just unresponsive. The look up takes a long duration and many times just time out. The solution? There is small hack on BSNL for this. Use third party DNS servers instead of BSNL DNS servers or run your own one like djbdns. The easiest options is to use OpenDNS. Just reconfigure your network to use the following DNS servers:


208.67.222.222
208.67.220.220


Detailed instructions specific to operating system or your BSNL modem are available in the OpenDNS website itself. After I reconfigured my BSNL modem to use the above 2 IP addresses, my DNS problems just vanished! Other ‘freebies’ that come with OpenDNS are phishing filters and automatic URL correction. Even if your service provider’s DNS servers are working fine, you can still use OpenDNS just for these two special features. After you hack BSNL DNS servers, you will see a noticeable improvement in your broadband speed.