Deep Dive into IP Subnetting and Subnet Masks, Binary to Decimal Conversion

Deep Dive into IP Subnetting and Subnet Masks

1. Introduction to IP Addressing

An IP address is a unique identifier for a device on a network. IPv4 addresses are 32-bit numbers, typically represented in dotted decimal notation (e.g., 192.168.1.1).

Each IP address consists of:

Network Portion – Identifies the network.
Host Portion – Identifies a specific device within the network.

To determine which part belongs to the network and which belongs to the host, we use a subnet mask.

2. Understanding Subnet Masks

A subnet mask is a 32-bit value that separates the network and host portions of an IP address.

It consists of contiguous ones (1s) for the network part and zeros (0s) for the host part.
Example of a subnet mask:

255.255.255.0

In binary:

11111111.11111111.11111111.00000000

The ones (1s) define the network.
The zeros (0s) define the hosts.

This means all IP addresses in the subnet share the same network part, while the host part varies.

3. Converting Dotted Decimal to Binary

Since IP addresses and subnet masks are actually binary, let's break it down.

Example:

Convert 255.255.255.252 to binary:

Decimal

Binary

255

11111111

255

11111111

255

11111111

252

11111100

Subnet mask in binary:

11111111.11111111.11111111.11111100

Here, 30 bits are network bits, and 2 bits are host bits.

4. Finding the Number of Networks and Hosts

We use these formulas:

Number of Hosts per Subnet

2h−22^h - 22h−2

Where:

hhh = number of host bits (0s in the subnet mask).
Subtract 2 because: The first address is reserved as the network address. The last address is reserved as the broadcast address.

Example (255.255.255.252 or /30)

Subnet mask: 11111111.11111111.11111111.11111100
Host bits: 2
Hosts per subnet: 22−2=4−2=2 usable hosts2^2 - 2 = 4 - 2 = 2 \text{ usable hosts}22−2=4−2=2 usable hosts (Perfect for a point-to-point link!)

Number of Subnets Created

2s2^s2s

Where:

sss = number of subnet bits (1s beyond the default network mask).

Example (Splitting a /24 into /30 subnets)

Original /24 mask: 255.255.255.0 → 11111111.11111111.11111111.00000000
New /30 mask: 255.255.255.252 → 11111111.11111111.11111111.11111100
Extra subnet bits: 30 - 24 = 6
Total subnets: 26=642^6 = 6426=64

So, a /30 subnetting scheme divides a /24 network into 64 subnets.

5. Example of Subnetting a Class C Network

Assume we have 192.168.1.0/24, and we want to subnet it into /26 subnets.

Step 1: Determine New Subnet Mask

/24 Mask: 255.255.255.0
/26 Mask: 255.255.255.192

11111111.11111111.11111111.11000000

Step 2: Identify the Changes

Subnet bits: 26 - 24 = 2
New subnets: 22=42^2 = 422=4 subnets
Host bits: 6
Usable hosts per subnet: 26−2=622^6 - 2 = 6226−2=62

Step 3: List the Subnet Ranges

Since we borrowed 2 bits, our subnet increments by 64 in the last octet:

192.168.1.0/26 → 192.168.1.0 - 192.168.1.63
192.168.1.64/26 → 192.168.1.64 - 192.168.1.127
192.168.1.128/26 → 192.168.1.128 - 192.168.1.191
192.168.1.192/26 → 192.168.1.192 - 192.168.1.255

Each subnet has 62 usable hosts.

6. CIDR Notation and VLSM

CIDR (Classless Inter-Domain Routing) allows flexible subnetting by defining subnet masks with a slash (/) notation.
VLSM (Variable-Length Subnet Masking) allows different-sized subnets within the same network, optimizing address usage.

Example:

/30 for point-to-point links (2 hosts).
/26 for office LANs (62 hosts).
/24 for larger subnets (254 hosts).

7. Summary of Important Points

Subnet Mask

CIDR

Host Bits

Usable Hosts

Subnets from /24

255.255.255.0 (/24) 11111111.11111111.11111111.0 = 8bits.8bits.8.bit.0bits =24bits

8. Conclusion

Subnetting breaks a large network into smaller subnets.
Subnet masks define network vs. host portions.
Binary calculations help determine available subnets and hosts.
Choosing the right subnet size ensures efficient IP allocation.

================================================

Binary to Decimal and Vice Versa in IP Subnetting

--------------------------------------------------------------------------------

In subnetting, IP addresses and subnet masks are represented in both binary and decimal. To understand how subnets work, it's crucial to know how to convert between binary and decimal representations.

1. Understanding IPv4 Address Representation

----------------------------------------------------------------------------

An IPv4 address consists of four octets (32 bits total), separated by dots. Each octet is an 8-bit binary number that can be converted to decimal.

For example, the IP address 192.168.1.1 in binary:

11000000.10101000.00000001.00000001

Similarly, subnet masks follow the same rule:

255.255.255.0 → 11111111.11111111.11111111.00000000

2. Converting Binary to Decimal in Subnetting

-------------------------------------------------------------------------

To convert a binary IP address or subnet mask to decimal, break each 8-bit octet and convert it separately.

Example 1: Convert 11000000.10101000.00000001.00000001 (Binary IP) to Decimal

Break it into four octets:

11000000 → 192

10101000 → 168

00000001 → 1

Decimal Equivalent: 192.168.1.1

Example 2: Convert 11111111.11111111.11111111.00000000 (Subnet Mask) to Decimal

11111111 → 255

00000000 → 0

Decimal Equivalent: 255.255.255.0

3. Converting Decimal to Binary in Subnetting

------------------------------------------------------------------------

To convert a decimal IP address or subnet mask to binary, convert each octet separately.

Example 3: Convert 172.16.254.1 (IP Address) to Binary

Convert each octet to 8-bit binary:

Decimal

Binary

172

10101100

00010000

254

11111110

00000001

Binary Equivalent: 10101100.00010000.11111110.00000001

Example 4: Convert 255.255.255.252 (Subnet Mask) to Binary

Decimal

Binary

255

11111111

255

11111111

255

11111111

252

11111100

Binary Equivalent: 11111111.11111111.11111111.11111100

4. Understanding CIDR Notation (/x) in Binary

-----------------------------------------------------------------------

CIDR (Classless Inter-Domain Routing) notation represents subnet masks using a slash (/) followed by the number of 1s in the mask.

Example:

/24 = 11111111.11111111.11111111.00000000 (255.255.255.0)
/30 = 11111111.11111111.11111111.11111100 (255.255.255.252)

Example 5: Convert /27 (CIDR Notation) to Decimal Subnet Mask

11111111.11111111.11111111.11100000

Breaking into decimal:

255.255.255.224

Thus, /27 = 255.255.255.224.

5. Determining Number of Hosts and Subnets Using Binary

---------------------------------------------------------------------------------------------

The number of host bits (0s) determines the number of usable hosts, and the number of borrowed bits (subnet bits) determines the number of subnets.

Formula for Hosts per Subnet:

2h−22^h - 22h−2

(where hhh = number of host bits)

Formula for Number of Subnets:

2s2^s2s

(where sss = number of subnet bits beyond the default network mask)

Example 6: Calculating Hosts for a /30 Subnet (255.255.255.252)

Binary Subnet Mask:

11111111.11111111.11111111.11111100

Host Bits: 2
Usable Hosts: 22−2=4−2=2 (Only 2 usable hosts per subnet)2^2 - 2 = 4 - 2 = 2 \text{ (Only 2 usable hosts per subnet)}22−2=4−2=2 (Only 2 usable hosts per subnet)

Example 7: Finding the Number of /27 Subnets in a /24 Network

/24 Mask: 255.255.255.0 → 11111111.11111111.11111111.00000000
/27 Mask: 255.255.255.224 → 11111111.11111111.11111111.11100000
Subnet Bits Borrowed: 27 - 24 = 3
Subnets Created: 23=8 (We now have 8 subnets instead of 1)2^3 = 8 \text{ (We now have 8 subnets instead of 1)}23=8 (We now have 8 subnets instead of 1)

6. Quick Reference Table for Subnets

----------------------------------------------------------

CIDR

Subnet Mask

Hosts per Subnet

Subnets from /24

/24

255.255.255.0

254

/25

255.255.255.128

126

/26

255.255.255.192

/27

255.255.255.224

/28

255.255.255.240

/29

255.255.255.248

/30

255.255.255.252

7. Summary

----------------------

Binary helps in subnetting calculations by allowing clear separation of network and host portions.
Subnet masks define network size, and CIDR notation simplifies representation.
Using binary conversions, we can determine usable hosts and subnets.

0 comments