Subnetting without calculator
Have you ever think, how do people tell end result of subnetting verbally without using any calculator i.e. ipcalc, online sites like http://cidr.xyz/, http://vlsm-calc.net/ etc.
Sometimes “LESS IS MORE” so I came up with a short blog to calculate subnetting quickly by assuming that people reading this must be having a basic knowledge of terms used in subnetting.
Let’s take an example of CIDR 192.168.0.0/24. Here this slash notation simply means that 24 of 32 bits are network bits. So let’s focus on /22, /23, /24 slash notation in below images and try to get IP and subnet mask against them quickly.
You must have seen cheat tables of subnet mask like given below :
Quick Method
Get the size of the network (including network and broadcast-address). For networks up to /24, this will be “2^(32-number-of-prefix-bits)”, in case of a /27 network 2^(32–27)=2⁵=32. Now build the difference between 256 and this number: 256–32=224
In the same way for /22, /23, /24 slash notation and refer Image: 2 to cross check your answers.
CIDR: 1st Octet . 2nd Octet . 3rd Octet . 4th Octet/slash notation
/32: 256–2⁰=256–1=255
/31: 256–2¹=256–2=254
/30: 256–2²=256–4=252
/29: 256–2³=256–8=248
…
…
/24: 256–2⁸=256-256=0
/24 #NetMask = 255.255.255.0
/23: 256–2⁹ or 256–2⁸*2¹ means will span across two octets
4th octet: 256–2⁸=256–256=0
3rd octet: 256–2¹=256–2=254
/23 #NetMask = 255.255.254.0
/22: 256–2¹⁰ or 256–2⁸*2² means will span across two octets
4th octet: 256–2⁸=256=256=0
3rd octet: 256–2²=256–4=252
/22 #NetMask = 255.255.252.0
Calculation of Number of IP
Quick Method
Always exclude 2 IPs from total number of IP as these are reserved for network and broadcast in every network. Again in case of a /27 network 2^(32–27)-2 = 2⁵-2 = 32–2 = 30
That means we will be getting 30 IPs from CIDR with slash notation /27
Result:
Network: 192.168.0.0/27
HostMin: 192.168.0.1
HostMax: 192.168.0.30
Broadcast: 192.168.0.31
Hosts/Net: 30
Now apply the same trick on /22, /23, /24 slash notation and cross check your answers with image: 2
Now we are all set to calculate number of IP and subnetting value without calculator so take a chill pill and forget about ipcalc and other tools to calculate the same.
