Subnetting Guide

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This is my personal subnetting quick reference guide that I put together for the Network+.

How to Subnet

1. Determine number of subnet mask bits

   • Given prefix notation (`/n)
     • n = entire network mask
     • e.g. 192.168.1.0/26
   • Given subnet mask (255.x.y.z)
     • uses decimal value
     • e.g. class C 255.255.255.240

2. Determine number of subnets

3. Determine number of hosts per subnet

4. Determine the subnet IDs

   • Convert the subnet octet to decimal value
   • subtract that value from 256
   • that’s your starting subnet
   • each following subnet ID is the next doubling of that initial value
     • e.g., if 16, then will be 32, 48, 64

Equations

• Number of subnets =
  • = number of subnet bits
  • each power of 2 is double previous one

1	2	4	8	16	32	64	128	256

• Number of hosts per subnet =

  • = number of host bits
  • 2 IP addresses reserved for network IP and for broadcast IP

• Subnet Interval =

  • Each subnet is a multiple of this number
  • This number starts the second subnet
  • This is also = # of hosts per subnet
  • E.g.,
    • first subnet is always the initial network
      • 192.168.1.0
    • second would be

Decimal values for the number of bits set to 1 in an octet within a mask:

1	2	3	4	5	6	7	8
128	192	224	240	248	252	254	255

Subnet Cheat Sheet

Subnets	1	2	4	8	16	32	64	128	256
Hosts	256	128	64	32	16	8	4	2	1
Decimal Mask	0	128	192	224	240	248	252	254	255
Subnet bits	0	1	2	3	4	5	6	7	8

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• Subnet practice: Click here