Networking - Subnet Masks and CIDR in networking

1. What is a Subnet Mask?

A subnet mask is used in IP addressing to separate the network portion and the host portion of an IP address.

Key Points

  • An IP address = Network ID + Host ID

  • Subnet mask determines which part is for the network and which part is for hosts.

  • Written in dotted decimal format (e.g., 255.255.255.0).

Example

IP Address: 192.168.1.10
Subnet Mask: 255.255.255.0

Octet Binary Meaning
192 11000000 Network
168 10101000 Network
1 00000001 Network
10 00001010 Host

Explanation:

  • 255.255.255.0 in binary → 11111111.11111111.11111111.00000000

  • 1's → Network part

  • 0's → Host part

So, here:

  • Network ID = 192.168.1.0

  • Host Range = 192.168.1.1 → 192.168.1.254

  • Broadcast Address = 192.168.1.255

2. Subnetting

Subnetting is dividing a big network into smaller networks for better management and security.

Example:
If you have 192.168.1.0/24 with 254 hosts, you can divide it into four subnets of 62 hosts each using 255.255.255.192.

3. What is CIDR (Classless Inter-Domain Routing)?

CIDR notation is a compact way to represent an IP address and its subnet mask.

Format

IP Address / Prefix Length

  • Example: 192.168.1.10/24

  • /24 → means first 24 bits are network bits.

  • Equivalent Subnet Mask: 255.255.255.0

CIDR Examples

CIDR Subnet Mask Network Bits Host Bits No. of Hosts
/8 255.0.0.0 8 24 16,777,214
/16 255.255.0.0 16 16 65,534
/24 255.255.255.0 24 8 254
/30 255.255.255.252 30 2 2

Formula:
Hosts per Subnet = 2^(Host bits) - 2
(Subtract 2 for network and broadcast addresses)

4. Difference Between Subnet Mask and CIDR

Aspect Subnet Mask CIDR
Format Dotted decimal Slash notation
Example 255.255.255.0 /24
Purpose Separates network & host bits Represents network size
Usage Traditional addressing Modern addressing

5. Example Calculation

Scenario:
You are given 192.168.10.0/26.

Step 1 → Subnet Mask

  • /26 → 26 network bits → Subnet Mask = 255.255.255.192

Step 2 → Number of Hosts

  • Host bits = 32 - 26 = 6

  • Hosts = 2^6 - 2 = 62

Step 3 → Subnets

  • Original /24 had 256 IPs.

  • New /26 gives 256 ÷ 64 = 4 subnets.

Step 4 → Subnet Ranges

Subnet Network ID Host Range Broadcast
1 192.168.10.0 192.168.10.1 → 62 192.168.10.63
2 192.168.10.64 192.168.10.65 → 126 192.168.10.127
3 192.168.10.128 192.168.10.129 → 190 192.168.10.191
4 192.168.10.192 192.168.10.193 → 254 192.168.10.255

6. Diagram

   IP: 192.168.10.0/26
   Subnet Mask: 255.255.255.192
   ┌────────────────────────────┬────────────────────────────┬─────────────┐
   | Network Bits (26)         | Host Bits (6)              |   Example   |
   └────────────────────────────┴────────────────────────────┴─────────────┘
   Network ID: 192.168.10.0
   Host Range: 192.168.10.1 → 192.168.10.62
   Broadcast: 192.168.10.63