Basic Structure of IPv4 - Subnetting

IPv4, in short, is an addressing system that allows devices connected to the internet (computers, phones, etc.) to recognize each other. Let's now examine the structure of this addressing.

• An IPv4 address is a 32-bit number consisting of 4 sections (octets) of 8 bits each. (8 bits = 1 byte)
• For human readability, these bits are converted into decimal numbers from 0-255, separated by dots. This is called Dotted-Decimal Notation.
  • Example: 11000000.10101000.00000001.00000001 (binary - what computers understand) = 192.168.1.1 (decimal - what we understand)
• We perform this conversion using the following logic.
  • IP Address: 192.168.1.1
  • Method: Each bit represents a power of 2. The decimal equivalents of the 8 bits within an octet are as follows:

    

  
  To get the number we have, we find which values in the table we need to "use". We write 1 under each value we use and 0 under those we don't.
  • 128(1) - 64(1) - 32(0) - 16(0) - 8(0) - 4(0) - 2(0) - 1(0) which means 128+64=192
  • 128(1) - 64(0) - 32(1) - 16(0) - 8(1) - 4(0) - 2(0) - 1(0) which means 128+32+8=168

Network and Host ID

Every IPv4 address consists of two fundamental parts. A Network Mask is used to separate these parts.

• Network ID: Specifies which network the device is on. It's like the street name. The 1s in the network mask represent this part.
• Host ID: Identifies a specific device on that network. It's like the house number. The 0s in the network mask represent this part.

Example:

• IP Address: 192.168.1.10
• Network Mask: 255.255.255.0 , The binary equivalent of this mask is: 11111111.11111111.11111111.00000000
• Analysis: This mask indicates that the first three octets (the first 24 bits) are the Network ID, and the last octet (the last 8 bits) is the Host ID.
  • Network ID: 192.168.1.0
  • Host ID: 10

CIDR Notation

This provides a more flexible way to express the network mask we just mentioned by appending a / (slash) and the number of bits representing the network ID to the end of the IP address.

• 192.168.1.10/24 → Indicates that the first 24 bits are the network ID (i.e., the 255.255.255.0 mask).
• 10.50.0.0/8 → Indicates that the first 8 bits are the network ID (i.e., the 255.0.0.0 mask).

Special-Purpose IP Addresses

Certain IP addresses and ranges are reserved for special tasks and cannot be assigned to a normal device.

• Network Address: Defines the network itself. All bits in the host ID portion are 0 (e.g., 192.168.1.0/24).
• Broadcast Address: Used to send data to all devices on a network. All bits in the host ID portion are 1 (e.g., 192.168.1.255/24).
• Local Loopback: The address 127.0.0.1 is used for a device to communicate with itself. It is generally used for testing purposes.
• Private IP Ranges: These are addresses that cannot be routed on the public internet and are only for use on local networks (home, office, etc.).
  • 10.0.0.0 - 10.255.255.255 (10.0.0.0/8)
  • 172.16.0.0 - 172.31.255.255 (172.16.0.0/12)
  • 192.168.0.0 - 192.168.255.255 (192.168.0.0/16)

Subnetting

This is the process of dividing a large IP network into smaller, more efficient, and manageable Subnets. Let's walk through an example.

Step 1: Calculating Basic Information

• IP Address: 192.168.12.140/26
• CIDR: /26
• Network Mask: 255.255.255.192
• Binary Mask: 11111111.11111111.11111111.11000000

Now, let's find the fundamental addresses of the subnet with this information:

1. Network Address:
   • Found by setting all bits in the Host Portion of the IP address to 0.
   • IP (Binary): 11000000.10101000.00001100.10001100
   • Mask (Binary): 11111111.11111111.11111111.11000000
   • Result (Binary): 11000000.10101000.00001100.10000000
   • Result (Decimal): 192.168.12.128
2. Broadcast Address:
   • Found by setting all bits in the Host Portion of the IP address to 1.
   • Result (Binary): 11000000.10101000.00001100.10111111
   • Result (Decimal): 192.168.12.191
3. Usable IP Range:
   • The Network and Broadcast addresses cannot be assigned to devices. All addresses in between are usable.
   • Result: 192.168.12.129 - 192.168.12.190
4. Number of Usable Hosts:
   • A /26 mask leaves 32 - 26 = 6 bits for the host portion. The total number of IPs is 2^6 = 64.
   • Result: 64 - 2 (Network and Broadcast Address) = 62 usable IPs.

Step 2: Dividing the Network into Smaller Subnets

Let's divide the existing 192.168.12.128/26 network into 4 equal parts.

1. Finding the Required Number of Bits: To create 4 subnets, since 2^2 = 4, we need to "borrow" 2 more bits for the network mask.
   • The rule of thumb is: If you borrow n bits, you create 2ⁿ new subnets.
   Word Wrap

   Exponent	Value
   2 ^0 - 00000000 - Full Network	= 1
   2 ^1 - 10000000 - 2 Networks	= 2
   2 ^2 - 11000000 - 4 Networks	= 4
   2 ^3 - 11100000	= 8
   2 ^4 - ....	= 16
   2 ^5	= 32
   2 ^6	= 64
   2 ^7	= 128
   2 ^8	= 256

2. Determining the New Subnet Mask: Our current mask was /26. We find the new mask by adding 2 more bits.
   • /26 + 2 bits = /28 (or 255.255.255.240)
3. Calculating the New Subnets: When our large block of 64 addresses is divided by 4, each new subnet will have 16 addresses (64 / 4 = 16). Starting from 192.168.12.128, we proceed in increments of 16:

• 1st Subnet: 192.168.12.128/28 (Network: .128, Broadcast: .143, Usable: .129 - .142)
• 2nd Subnet: 192.168.12.144/28 (Network: .144, Broadcast: .159, Usable: .145 - .158)
• 3rd Subnet: 192.168.12.160/28 (Network: .160, Broadcast: .175, Usable: .161 - .174)
• 4th Subnet: 192.168.12.176/28 (Network: .176, Broadcast: .191, Usable: .177 - .190)

Quick Subnet Calculation Method

More practical methods can be used instead of complex binary calculations.

1. Finding the Relevant Octet: Determine which octet you are working on by looking at the CIDR mask.
   • /8 - /15 → 2nd octet
   • /16 - /23 → 3rd octet
   • /24 - /30 → 4th octet
   
   By remembering these four numbers, we will determine which octet of the IP Address can change.
   • For example, given the network address 192.168.1.1/25, it is immediately clear that 192.168.2.4 is not on the same network because a /25 subnet means only the fourth octet can change.
2. Finding the Block Sizes: Find the increment interval for subnets in the relevant octet.
   • Formula: 256 - (Mask Value in the Relevant Octet)
   • Example ( for /26): Mask is 255.255.255.192. The value of the relevant octet (4th) is 192.
     • Block Size: 256 - 192 = 64. This indicates that the network addresses will increment by 64 (0, 64, 128, 192).
   • Example (for /28): Mask is 255.255.255.240. The value of the relevant octet (4th) is 240.
     • Block Size: 256 - 240 = 16. This indicates that the network addresses will increment by 16 (0, 16, 32, ... 128, 144, 160...).

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