Subnetting

In computer networking, a subnet (short for "subnetwork") is a logical subdivision of an IP network. Subnetting allows you to divide a larger network into smaller, more manageable segments. This is done using a subnet mask, which defines the boundaries of the subnet.

Key Concepts:

1. IP Address: An IP address (e.g., 192.168.1.1) is a unique identifier for a device on a network. It consists of two parts:

   • Network portion: Identifies the network.

   • Host portion: Identifies the specific device within the network.

2. Subnet Mask: A subnet mask (e.g., 255.255.255.0) is used to determine which part of the IP address is the network portion and which part is the host portion. It is a 32-bit number, typically written in dotted-decimal notation (like an IP address).

   • The 1s in the subnet mask represent the network portion.

   • The 0s represent the host portion.

   For example:

   • IP Address: 192.168.1.1

   • Subnet Mask: 255.255.255.0

     • This means the first 24 bits are the network portion (192.168.1), and the last 8 bits are the host portion (1).

3. CIDR Notation: Subnets are often represented using Classless Inter-Domain Routing (CIDR) notation, which combines the IP address and the subnet mask into a single format. For example:

   • 192.168.1.0/24 means the subnet mask is 255.255.255.0 (24 bits for the network portion).

4. Subnetting: Subnetting involves borrowing bits from the host portion of the IP address to create smaller subnets. For example:

   • A /24 subnet (255.255.255.0) allows for 256 IP addresses (254 usable for hosts).

   • A /26 subnet (255.255.255.192) allows for 64 IP addresses (62 usable for hosts).

Example:

• IP Address: 192.168.1.0

• Subnet Mask: 255.255.255.0 (or /24 in CIDR notation)

  • Network Address: 192.168.1.0

  • Usable Host Range: 192.168.1.1 to 192.168.1.254

  • Broadcast Address: 192.168.1.255

Why Subnetting?

• Efficient use of IP addresses: Prevents wasting IP addresses.

• Improved network performance: Reduces congestion by limiting broadcast domains.

• Enhanced security: Isolates network segments.

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How to calculate the subnet mask:

Calculating a subnet mask involves determining how many bits are allocated to the network portion and how many are left for the host portion of an IP address. Here's a step-by-step guide to calculating a subnet mask:

1. Understand CIDR Notation

• CIDR (Classless Inter-Domain Routing) notation represents the subnet mask in the format IP Address/Prefix Length. For example:

  • 192.168.1.0/24 means the first 24 bits are the network portion, and the remaining 8 bits are for hosts.

  • The prefix length (/24) directly corresponds to the number of 1s in the subnet mask.

2. Convert Prefix Length to Subnet Mask

• A subnet mask is a 32-bit number, written in dotted-decimal notation (e.g., 255.255.255.0).

• Each 1 in the subnet mask represents a bit in the network portion, and each 0 represents a bit in the host portion.

Example:

• For /24:

  • The first 24 bits are 1s, and the last 8 bits are 0s.

  • In binary: 11111111.11111111.11111111.00000000

  • In decimal: 255.255.255.0

3. Use a Subnet Mask Chart

Here’s a quick reference chart for common subnet masks:

CIDR Prefix	Subnet Mask	Number of Usable Hosts
/24	255.255.255.0	254
/25	255.255.255.128	126
/26	255.255.255.192	62
/27	255.255.255.224	30
/28	255.255.255.240	14
/29	255.255.255.248	6
/30	255.255.255.252	2

4. Calculate Subnet Mask Manually

If you need to calculate a subnet mask for a specific number of hosts or subnets, follow these steps:

Step 1: Determine the Number of Hosts or Subnets

• Decide how many hosts or subnets you need.

• Use the formula:

  • Number of hosts = 2(32−Prefix Length)−22(32−Prefix Length)−2

  • Number of subnets = 2(Prefix Length−Original Prefix Length)2(Prefix Length−Original Prefix Length)

Step 2: Find the Prefix Length

• For hosts:

  • Calculate the number of host bits required: Host Bits=log⁡2(Number of Hosts+2)Host Bits=log2​(Number of Hosts+2).

  • Subtract the host bits from 32 to get the prefix length.

• For subnets:

  • Calculate the number of subnet bits required: Subnet Bits=log⁡2(Number of Subnets)Subnet Bits=log2​(Number of Subnets).

  • Add the subnet bits to the original prefix length.

Step 3: Convert Prefix Length to Subnet Mask

• Use the prefix length to determine the subnet mask (as shown in Step 2).

5. Example Calculations

Example 1: Calculate Subnet Mask for 60 Hosts

• You need at least 60 usable hosts.

• Use the formula: 2(32−Prefix Length)−2≥602(32−Prefix Length)−2≥60.

• Solve for the prefix length:

  • 2(32−Prefix Length)≥622(32−Prefix Length)≥62.

  • 32−Prefix Length=632−Prefix Length=6 (since 26=6426=64).

  • Prefix Length = 32−6=2632−6=26.

• Subnet Mask: /26 or 255.255.255.192.

Example 2: Calculate Subnet Mask for 4 Subnets

• You need 4 subnets from a /24 network.

• Use the formula: 2(Prefix Length−24)≥42(Prefix Length−24)≥4.

• Solve for the prefix length:

  • Prefix Length−24=2Prefix Length−24=2 (since 22=422=4).

  • Prefix Length = 24+2=2624+2=26.

• Subnet Mask: /26 or 255.255.255.192.

6. Tools for Subnetting

• Use online subnet calculators or tools like:

  • IP Subnet Calculator

  • SolarWinds Subnet Calculator