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# IP Address : Subnet Mask

An IP Address Have Two Parts, One Part Is Designated For A Network, The Remaining Is Designated For The Host. So The Way To Tell Which Portion Belongs To Either The Network Or The Host Is Where The Subnet Mask Comes In.

A Subnet Mask Is A Number That Resembles An IP Address.

It Reveals How Many Bits In The IP Address Used For The Network.

By Masking The Network Portion Of The IP Address.

IP Address:    173 . 16 . 0 . 0

Subnet mask: 255 . 255 . 0 . 0

So In This Subnet Mask, The First Two Octets Are 255

And We Would Look At This Subnet Mask In Binary Form The First Two Octets Will Be All One’s.

255.255.0.0 — 11111111.11111111.00000000.00000000

Because When You Count All The Numbers In An Octet, It Will Equal 255.

Example – IP Address 173.16.0.0 with its subnet mask

Binary Form – 10101101.00010000.00000000.00000000

255.255.0.0 – 11111111.11111111.00000000.00000000

So The Way To Tell Which Portion Of This IP Address Is A Network Portion

Is When The Subnet Mask Binary Digit Is 1, It Will Indicate The Position Of The IP Address

That Defines The Network.

So We’ll Cross Up All The Digits In The IP Address That Line Up With 1’s In The Subnet Mask.

And When You Do This It Will Reveal That The First Two Octets Are The Network Portion.

And The Remaining Is The Host Portion.

Here Is The Chart For The Default Subnet Mask For Class A,B,C Ip Address.

Notice The Change Of The Location Of The Network And Host Portions Between Them.

You Are Going To Need To Memorize The Default Subnet Mask Classes For The Exam.

## IPCLASSES AND RANGES

IP Addresses Are Assigned To Different Organisations In Blocks.

These Blocks Are Divided Into 5 Classes

But For The Exam, You Only Need To Know 3 of Them.

They Are Class A, Class B, Class C

You Can Tell By The Number In The First Octet Which Class An IP Address Belongs To.

So

Class A IP Ranges From 1-126

This Class Supports 16 Million Hosts On 126 Networks.

So This Class Is Mainly Given To Large Organisations Because Of THe Tremendous Amount Of IP Addresses It Can Give Out.

Class B IP Range Is From 128-191

This Class Supports 65000 Hosts On 16000 Networks.

This Class Is Given To Medium Size Organisations.

Class C IP Ranges From 192-223

This Class Supports 254 Hosts On 2 Million Networks

This Class Is Given To Small Organisations.

Now If You Notice We Escaped The Over The 127 Range

This Number Cannot Be Used For A Network Because 127 Address Is Reserved For Internal Loopback Fuctions.

You Are Also Going To Need To Memorize The Default Subnet Masks For Each Of These Classes.