IP Addressing and Subnetting Simplified

N

Naval Kishor Upadhyay

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At first, IP addresses and subnetting look difficult, full of numbers and strange masks. But in reality, they are just a way to organize devices, like dividing houses into streets. Let’s go step by step with simple examples.

What Is an IP Address?​


An IP address is like a digital house number for a device on a network.

Example:


Code: 
192.168.1.10

It has two parts:

  • Network part β†’ like the street name.
  • Host part β†’ like the house number on that street.

Subnet Masks and CIDR​


The subnet mask tells us how many bits belong to the network and how many are left for hosts.

Example:


Code: 
IP Address: 192.168.1.10
Subnet Mask: 255.255.255.0
CIDR: /24
  • /24 means: first 24 bits are for the network.
  • This leaves 8 bits for hosts β†’ 2^8 = 256 addresses.
  • From those, 254 are usable (2 are reserved: network and broadcast).

So 192.168.1.0/24 can have 254 devices.

Why Subnetting?​


Subnetting is like dividing a big apartment block into smaller buildings. Instead of one very large network, you create smaller, more organized networks.

Example:

You have 192.168.1.0/24 (254 devices possible). But your company has three teams:

  • HR β†’ needs 50 devices
  • IT β†’ needs 50 devices
  • Sales β†’ needs 100 devices

If everyone stays in one flat network, there will be chaos (too many devices shouting in the same broadcast domain).

By subnetting, you split into smaller groups.

Simple Subnet Example: /25​


Let’s take 192.168.1.0/24 and cut it in half.

  • Subnet mask: /25 = 255.255.255.128

Why does /25 equal 255.255.255.128?​

  • /25 means the first 25 bits of the 32-bit address are used for the network.
  • In binary, the mask looks like this:

Code: 
11111111.11111111.11111111.10000000
  • Convert each 8-bit block into decimal:
    • 11111111 = 255
    • 11111111 = 255
    • 11111111 = 255
    • 10000000 = 128

So the mask is 255.255.255.128.

  • The remaining 7 bits are for hosts β†’ 2^7 = 128 addresses β†’ 126 usable.

Subnetting Result​

SubnetNetwork AddrHost RangeBroadcast Addr
192.168.1.0/25192.168.1.0192.168.1.1–126192.168.1.127
192.168.1.128/25192.168.1.128192.168.1.129–254192.168.1.255

So now you can put HR in the first subnet and Sales in the second.

Smaller Subnet Example: /26​


Now let’s cut /24 into 4 equal parts.

  • Subnet mask: /26 = 255.255.255.192

Why does /26 equal 255.255.255.192?​

  • /26 means the first 26 bits are for the network.
  • In binary, the mask looks like this:

Code: 
11111111.11111111.11111111.11000000
  • Convert each 8-bit block into decimal:
    • 11111111 = 255
    • 11111111 = 255
    • 11111111 = 255
    • 11000000 = 192

So the mask is 255.255.255.192.

  • The remaining 6 bits are for hosts β†’ 2^6 = 64 addresses β†’ 62 usable.

Subnetting Result​

SubnetHost RangeBroadcast Addr
192.168.1.0/26192.168.1.1–62192.168.1.63
192.168.1.64/26192.168.1.65–126192.168.1.127
192.168.1.128/26192.168.1.129–190192.168.1.191
192.168.1.192/26192.168.1.193–254192.168.1.255

Now each subnet can handle a small team of up to 62 devices.

Shortcut Rules (No Math Needed)​

  • /24 β†’ 256 total β†’ 254 usable
  • /25 β†’ 128 total β†’ 126 usable
  • /26 β†’ 64 total β†’ 62 usable
  • /30 β†’ 4 total β†’ 2 usable (for point-to-point links)

Why 2 Addresses Are Reserved?​


  • Network address β†’ the first address in the subnet. It represents the subnet itself. Routers use it in routing tables.
    • Example: in 192.168.1.0/24, the first address (192.168.1.0) is the network.

  • Broadcast address β†’ the last address in the subnet. It is used to send a message to all devices in that subnet.
    • Example: in 192.168.1.0/24, the last address (192.168.1.255) is the broadcast.

That’s why from 256 addresses in a /24, only 254 are usable for devices.

Visual Diagram of Subnetting​

/24 β†’ One Big Subnet​


Code: 
192.168.1.0 ---------------------------- 192.168.1.255
    (254 usable hosts in one network)

/25 β†’ Two Subnets​


Code: 
192.168.1.0 -------- 192.168.1.127   |   192.168.1.128 -------- 192.168.1.255
    (126 hosts)                           (126 hosts)

/26 β†’ Four Subnets​


Code: 
192.168.1.0 -- 63   |   192.168.1.64 -- 127   |   192.168.1.128 -- 191   |   192.168.1.192 -- 255
   (62 hosts)           (62 hosts)               (62 hosts)                   (62 hosts)

This way, you can see how subnetting divides a big block into smaller chunks.

Why It Matters​

  • Organizes networks β†’ like splitting a big city into smaller neighborhoods.
  • Improves performance β†’ less β€œnoise” from broadcast traffic.
  • Adds security β†’ different teams/devices can be separated.

Conclusion​


IP addressing is like giving houses numbers on streets. Subnetting is like dividing the city into districts. With just a few rules, you can design networks that are neat, scalable, and efficient.

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