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Subnetting

Chapter 4: Subnetting - Complete Mastery Guide

Section titled “Chapter 4: Subnetting - Complete Mastery Guide”

Subnetting is one of the most critical skills for any network administrator. This chapter provides an exhaustive explanation of subnetting from basics to advanced concepts.

4.1 Why Subnetting?

Section titled “4.1 Why Subnetting?”

Before diving into the mechanics, let’s understand why subnetting exists.

+------------------------------------------------------------------+
|                   Why Subnetting Matters                           |
+------------------------------------------------------------------+


Problem: IPv4 Address Exhaustion
+------------------------------------------------------------------+


Total IPv4 addresses: 4,294,967,296 (2^32)
- Too small for world population (~8 billion people)
- Not every address is usable (reservations, multicasts, etc.)
- Classful allocation was wasteful


The Solution: Subnetting
+------------------------------------------------------------------+


Subnetting allows us to:
- Divide large network into smaller networks
- Use addresses efficiently
- Improve performance (smaller broadcast domains)
- Enhance security (isolate segments)
- Simplify management


Without Subnetting:
+------------------------------------------------------------------+


Network 192.168.0.0/16
+------------------------------------------------------------------+
| 65,536 hosts | All in same broadcast domain                     |
| Performance degradation                                         |
| Security risk                                                    |
| No granular control                                             |
+------------------------------------------------------------------+


With Subnetting:
+------------------------------------------------------------------+


192.168.0.0/24   (254 hosts) - Engineering Dept
192.168.1.0/24   (254 hosts) - Sales Dept
192.168.2.0/24   (254 hosts) - HR Dept
192.168.3.0/24   (254 hosts) - Guest WiFi
192.168.4.0/24   (254 hosts) - Servers


+------------------------------------------------------------------+
Benefits:
- Isolated broadcast domains
- Better performance
- Improved security
- Easier management


+------------------------------------------------------------------+

4.2 Binary Math Fundamentals

Section titled “4.2 Binary Math Fundamentals”

Subnetting requires comfortable working with binary. Let’s master this.

+------------------------------------------------------------------+
|                   Binary Math Deep Dive                           |
+------------------------------------------------------------------+


Bits and Bytes:
+------------------------------------------------------------------+


1 bit   = 0 or 1
1 byte  = 8 bits = 0-255


Bit Position Values (Right to Left):
+------------------------------------------------------------------+


Position:  7   6   5   4   3   2   1   0
Value:     128 64  32  16  8   4   2   1


Converting Binary to Decimal:
+------------------------------------------------------------------+


Example: 10110100


= 128 + 0 + 32 + 16 + 0 + 4 + 0 + 0
= 128 + 32 + 16 + 4
= 180


Another: 01101001


= 0 + 64 + 32 + 0 + 8 + 0 + 0 + 1
= 64 + 32 + 8 + 1
= 105


Converting Decimal to Binary:
+------------------------------------------------------------------+


Method 1: Subtract Powers of Two


Example: 200


200 - 128 = 72
 72 - 64  = 8
  8 - 8   = 0


128 ✓  64 ✓  32 ✗  16 ✗  8 ✓  4 ✗  2 ✗  1 ✗
= 11001000


Method 2: Division by 2


200 ÷ 2 = 100 remainder 0
100 ÷ 2 = 50  remainder 0
50  ÷ 2 = 25  remainder 0
25  ÷ 2 = 12  remainder 1
12  ÷ 2 = 6   remainder 0
6   ÷ 2 = 3   remainder 0
3   ÷ 2 = 1   remainder 1
1   ÷ 2 = 0   remainder 1


Read remainders from bottom: 11001000


Practice Chart:
+------------------------------------------------------------------+


| Decimal | Binary   |
|---------|----------|
| 0       | 00000000 |
| 1       | 00000001 |
| 2       | 00000010 |
| 4       | 00000100 |
| 8       | 00001000 |
| 16      | 00010000 |
| 32      | 00100000 |
| 64      | 01000000 |
| 128     | 10000000 |
| 255     | 11111111 |


Powers of 2:
+------------------------------------------------------------------+


| 2^0  = 1    | 2^8  = 256    |
| 2^1  = 2    | 2^9  = 512    |
| 2^2  = 4    | 2^10 = 1024   |
| 2^3  = 8    | 2^11 = 2048   |
| 2^4  = 16   | 2^12 = 4096   |
| 2^5  = 32   | 2^13 = 8192   |
| 2^6  = 64   | 2^14 = 16384  |
| 2^7  = 128  | 2^15 = 32768  |


Quick Formulas:
+------------------------------------------------------------------+


Hosts per subnet = 2^(host bits) - 2
Subnets created = 2^(new subnet bits)


Example: /24 to /26
- Borrowed 2 bits
- New subnets = 2^2 = 4
- Hosts per subnet = 2^6 - 2 = 62


+------------------------------------------------------------------+

4.3 IP Address Classes

Section titled “4.3 IP Address Classes”

Understanding the historical class system helps understand subnetting.

+------------------------------------------------------------------+
|               IPv4 Address Classes (Historical)                    |
+------------------------------------------------------------------+


Classful Addressing (1981-1993):
+------------------------------------------------------------------+


Class A:
+------------------------------------------------------------------+
| First bit: 0                                                     |
| Range: 1.0.0.0 - 126.255.255.255                                |
| Default mask: /8 (255.0.0.0)                                    |
| Networks: 126 (2^7 - 2)                                         |
| Hosts per network: 16,777,214 (2^24 - 2)                        |
| Examples: 10.0.0.0, 3.0.0.0, 55.0.0.0                         |
+------------------------------------------------------------------+


Class B:
+------------------------------------------------------------------+
| First bits: 10                                                   |
| Range: 128.0.0.0 - 191.255.255.255                             |
| Default mask: /16 (255.255.0.0)                                 |
| Networks: 16,382 (2^14 - 2)                                     |
| Hosts per network: 65,534 (2^16 - 2)                           |
| Examples: 172.16.0.0, 169.254.0.0                             |
+------------------------------------------------------------------+


Class C:
+------------------------------------------------------------------+
| First bits: 110                                                 |
| Range: 192.0.0.0 - 223.255.255.255                            |
| Default mask: /24 (255.255.255.0)                              |
| Networks: 2,097,150 (2^21 - 2)                                 |
| Hosts per network: 254 (2^8 - 2)                                |
| Examples: 192.168.0.0, 208.0.0.0                               |
+------------------------------------------------------------------+


Class D (Multicast):
+------------------------------------------------------------------+
| First bits: 1110                                                |
| Range: 224.0.0.0 - 239.255.255.255                            |
| Not for host addressing                                         |
| Examples: 224.0.0.5 (OSPF), 224.0.0.9 (RIP2)                   |
+------------------------------------------------------------------+


Class E (Reserved):
+------------------------------------------------------------------+
| First bits: 1111                                                |
| Range: 240.0.0.0 - 255.255.255.255                            |
| Reserved for future use                                          |
| 255.255.255.255 is limited broadcast                            |
+------------------------------------------------------------------+


Special IP Addresses:
+------------------------------------------------------------------+


| Address | CIDR        | Purpose                                    |
|---------|-------------|--------------------------------------------|
| 0.0.0.0/8 | 0.0.0.0   | "This network" (RFC 1122)               |
| 10.0.0.0/8 | 10.0.0.0 | Private (RFC 1918)                       |
| 127.0.0.0/8 | 127.0.0.0 | Loopback                                 |
| 169.254.0.0/16 | 169.254.0.0 | Link-local (APIPA)           |
| 172.16.0.0/12 | 172.16.0.0 | Private (RFC 1918)             |
| 192.0.0.0/24 | 192.0.0.0 | IETF Protocol (RFC 5737)           |
| 192.0.2.0/24 | 192.0.2.0 | TEST-NET-1 (RFC 5737)               |
| 192.168.0.0/16 | 192.168.0.0 | Private (RFC 1918)              |
| 198.51.100.0/24 | 198.51.100.0 | TEST-NET-2 (RFC 5737)        |
| 203.0.113.0/24 | 203.0.113.0 | TEST-NET-3 (RFC 5737)         |
| 255.255.255.255/32 | 255.255.255.255 | Limited Broadcast |


Private Address Ranges (RFC 1918):
+------------------------------------------------------------------+


10.0.0.0/8       - 10.255.255.255 (16,777,216 addresses)
172.16.0.0/12     - 172.31.255.255 (1,048,576 addresses)
192.168.0.0/16    - 192.168.255.255 (65,536 addresses)


+------------------------------------------------------------------+

4.4 Subnet Mask Deep Dive

Section titled “4.4 Subnet Mask Deep Dive”
+------------------------------------------------------------------+
|                   Subnet Mask Deep Dive                           |
+------------------------------------------------------------------+


What is a Subnet Mask?
+------------------------------------------------------------------+


A subnet mask is a 32-bit number that divides an IP address into:
- Network portion
- Host portion


The mask has 1s for network bits and 0s for host bits.


Example: 255.255.255.0
+------------------------------------------------------------------+


Binary: 11111111.11111111.11111111.00000000
         |--------Network---------|---Hosts--|


Network bits: 24
Host bits: 8


With IP 192.168.1.10:
+------------------------------------------------------------------+


IP:       11000000.10101000.00000001.00001010
Mask:     11111111.11111111.11111111.00000000
          ----------------------------------------
AND:      11000000.10101000.00000001.00000000


Network: 192.168.1.0
Host:    10


Common Subnet Masks:
+------------------------------------------------------------------+


| CIDR   | Mask            | Hosts    | Common Use                   |
|--------|-----------------|----------|------------------------------|
| /32    | 255.255.255.255| 0        | Single host                 |
| /31    | 255.255.255.254| 0*       | Point-to-point links        |
| /30    | 255.255.255.252| 2        | WAN links                   |
| /29    | 255.255.255.248| 6        | Small segment               |
| /28    | 255.255.255.240| 14       | Small network               |
| /27    | 255.255.255.224| 30       | Small office                |
| /26    | 255.255.255.192| 62       | Department                  |
| /25    | 255.255.255.128| 126      | Large department            |
| /24    | 255.255.255.0  | 254      | Standard subnet             |
| /23    | 255.255.254.0  | 510      | Campus segment              |
| /22    | 255.255.252.0  | 1022     | Building                    |
| /21    | 255.255.248.0  | 2046     | Large building              |
| /20    | 255.255.240.0  | 4094     | Small campus                |
| /19    | 255.255.224.0  | 8190     | Campus                      |
| /18    | 255.255.192.0  | 16382    | Large campus                |
| /17    | 255.255.128.0  | 32766    | City network                |
| /16    | 255.255.0.0    | 65534    | Organization                |
| /15    | 255.254.0.0    | 131070   | ISP                         |
| /8     | 255.0.0.0      | 16777214 | Large organization         |


* /31 is special (RFC 3021)


All 32 Bits:
+------------------------------------------------------------------+


/32: 11111111.11111111.11111111.11111111 (255.255.255.255)
/31: 11111111.11111111.11111111.11111110 (255.255.255.254)
/30: 11111111.11111111.11111111.11111100 (255.255.255.252)
/29: 11111111.11111111.11111111.11111000 (255.255.255.248)
/28: 11111111.11111111.11111111.11110000 (255.255.255.240)
/27: 11111111.11111111.11111111.11100000 (255.255.255.224)
/26: 11111111.11111111.11111111.11000000 (255.255.255.192)
/25: 11111111.11111111.11111111.10000000 (255.255.255.128)
/24: 11111111.11111111.11111111.00000000 (255.255.255.0)
/23: 11111111.11111111.11111110.00000000 (255.255.254.0)
/22: 11111111.11111111.11111100.00000000 (255.255.252.0)
/21: 11111111.11111111.11111000.00000000 (255.255.248.0)
/20: 11111111.11111111.11110000.00000000 (255.255.240.0)
/19: 11111111.11111111.11100000.00000000 (255.255.224.0)
/18: 11111111.11111111.11000000.00000000 (255.255.192.0)
/17: 11111111.11111111.10000000.00000000 (255.255.128.0)
/16: 11111111.11111111.00000000.00000000 (255.255.0.0)


+------------------------------------------------------------------+

4.5 Subnetting Examples - Step by Step

Section titled “4.5 Subnetting Examples - Step by Step”

Example 1: Simple /24 Subnetting

Section titled “Example 1: Simple /24 Subnetting”
+------------------------------------------------------------------+
|          Example 1: Subnetting 192.168.1.0/24                    |
+------------------------------------------------------------------+


Original: 192.168.1.0/24 (254 hosts)


We want to create 4 subnets (each with 62 hosts)


Step 1: Determine new CIDR
+------------------------------------------------------------------+


Need: 4 subnets = 2^2 = need 2 borrowed bits
Original /24 + 2 = /26


New CIDR: /26


Step 2: Calculate block size
+------------------------------------------------------------------+


Block size = 256 - mask
= 256 - 192 = 64


Block size = 64


Step 3: List subnets
+------------------------------------------------------------------+


Subnet 1: 192.168.1.0/26
  Range: 192.168.1.0 - 192.168.1.63
  Usable: 192.168.1.1 - 192.168.1.62 (62 hosts)
  Broadcast: 192.168.1.63


Subnet 2: 192.168.1.64/26
  Range: 192.168.1.64 - 192.168.1.127
  Usable: 192.168.1.65 - 192.168.1.126 (62 hosts)
  Broadcast: 192.168.1.127


Subnet 3: 192.168.1.128/26
  Range: 192.168.1.128 - 192.168.1.191
  Usable: 192.168.1.129 - 192.168.1.190 (62 hosts)
  Broadcast: 192.168.1.191


Subnet 4: 192.168.1.192/26
  Range: 192.168.1.192 - 192.168.1.255
  Usable: 192.168.1.193 - 192.168.1.254 (62 hosts)
  Broadcast: 192.168.1.255


Visual:
+------------------------------------------------------------------+


/26 blocks within /24:


| Subnet     | First IP   | Last IP    | Broadcast  | Usable |
|------------|------------|------------|------------|--------|
| .0/26      | .1         | .62        | .63        | 62     |
| .64/26     | .65        | .126       | .127       | 62     |
| .128/26    | .129       | .190       | .191       | 62     |
| .192/26     | .193       | .254       | .255       | 62     |


Total usable hosts: 62 × 4 = 248


+------------------------------------------------------------------+

Example 2: Variable Length Subnet Masking (VLSM)

Section titled “Example 2: Variable Length Subnet Masking (VLSM)”
+------------------------------------------------------------------+
|          Example 2: VLSM - Variable Length Subnet Masking        |
+------------------------------------------------------------------+


Network: 192.168.1.0/24


Requirement:
+------------------------------------------------------------------+


- Building A: 100 hosts (need /25, 126 hosts)
- Building B: 60 hosts (need /26, 62 hosts)
- Building C: 30 hosts (need /27, 30 hosts)
- Point-to-point links: 3 links (need /30, 2 hosts each)
- Management: 10 hosts (need /28, 14 hosts)


Step 1: Sort by size (largest first)
+------------------------------------------------------------------+


1. Building A: 100 hosts -> /25 (126 hosts)
2. Building B: 60 hosts -> /26 (62 hosts)
3. Building C: 30 hosts -> /27 (30 hosts)
4. Management: 10 hosts -> /28 (14 hosts)
5. Links: 3 × /30


Step 2: Allocate from main block
+------------------------------------------------------------------+


Main: 192.168.1.0/24 (256 addresses)


1. Building A: /25 (126 hosts)
   +------------------------------------------------------------------+
   | Subnet: 192.168.1.0/25                                         |
   | Range: 192.168.1.0 - 192.168.1.127                            |
   | Usable: 192.168.1.1 - 192.168.1.126 (126 hosts)             |
   +------------------------------------------------------------------+


2. Building B: /26 (62 hosts)
   +------------------------------------------------------------------+
   | Next available: 192.168.1.128                                  |
   | Subnet: 192.168.1.128/26                                      |
   | Range: 192.168.1.128 - 192.168.1.191                         |
   | Usable: 192.168.1.129 - 192.168.1.190 (62 hosts)            |
   +------------------------------------------------------------------+


3. Building C: /27 (30 hosts)
   +------------------------------------------------------------------+
   | Next available: 192.168.1.192                                  |
   | Subnet: 192.168.1.192/27                                      |
   | Range: 192.168.1.192 - 192.168.1.223                         |
   | Usable: 192.168.1.193 - 192.168.1.222 (30 hosts)            |
   +------------------------------------------------------------------+


4. Management: /28 (14 hosts)
   +------------------------------------------------------------------+
   | Next available: 192.168.1.224                                  |
   | Subnet: 192.168.1.224/28                                      |
   | Range: 192.168.1.224 - 192.168.1.239                         |
   | Usable: 192.168.1.225 - 192.168.1.238 (14 hosts)            |
   +------------------------------------------------------------------+


5. Point-to-point links: /30 (2 hosts each)
   +------------------------------------------------------------------+
   | Link 1: 192.168.1.240/30 (192.168.1.240 - 192.168.1.243)     |
   |   Usable: .241, .242 (2 hosts)                                |
   |   Broadcast: .243                                              |
   |                                                                  |
   | Link 2: 192.168.1.244/30 (192.168.1.244 - 192.168.1.247)     |
   |   Usable: .245, .246                                          |
   |                                                                  |
   | Link 3: 192.168.1.248/30 (192.168.1.248 - 192.168.1.251)     |
   |   Usable: .249, .250                                          |
   +------------------------------------------------------------------+


Summary Table:
+------------------------------------------------------------------+


| Subnet           | CIDR    | Hosts   | Range               |
|------------------|---------|---------|---------------------|
| Building A      | /25     | 126     | .0 - .127          |
| Building B      | /26     | 62      | .128 - .191        |
| Building C      | /27     | 30      | .192 - .223        |
| Management      | /28     | 14      | .224 - .239        |
| Link 1          | /30     | 2       | .240 - .243        |
| Link 2          | /30     | 2       | .244 - .247        |
| Link 3          | /30     | 2       | .248 - .251        |


Total used: 126 + 62 + 30 + 14 + 2 + 2 + 2 = 238
Remaining: 256 - 238 = 18 addresses (reserved)


+------------------------------------------------------------------+

4.6 Supernetting (CIDR)

Section titled “4.6 Supernetting (CIDR)”

Supernetting combines smaller networks into larger ones - the opposite of subnetting.

+------------------------------------------------------------------+
|                   Supernetting Deep Dive                         |
+------------------------------------------------------------------+


What is Supernetting?
+------------------------------------------------------------------+


Supernetting (also called CIDR - Classless Inter-Domain Routing):
- Combines multiple smaller networks into a larger network
- Reduces routing table entries (route summarization)
- Opposite of subnetting


Example:
+------------------------------------------------------------------+


4 × /24 networks can be summarized as /22


192.168.0.0/24   = 192.168.0.0 - 192.168.0.255
192.168.1.0/24   = 192.168.1.0 - 192.168.1.255
192.168.2.0/24   = 192.168.2.0 - 192.168.2.255
192.168.3.0/24   = 192.168.3.0 - 192.168.3.255
---------------------------------------------------------------
Combined:        = 192.168.0.0 - 192.168.3.255
= 192.168.0.0/22


How to Summarize:
+------------------------------------------------------------------+


Step 1: List networks in binary


192.168.0.0 = 11000000.10101000.00000000.00000000
192.168.1.0 = 11000000.10101000.00000001.00000000
192.168.2.0 = 11000000.10101000.00000010.00000000
192.168.3.0 = 11000000.10101000.00000011.00000000


Step 2: Find common bits


11000000.10101000.000000xx.xxxxxxxx
         |-------Common-------|


22 common bits = /22


Step 3: Write summarized route


192.168.0.0/22


Supernetting Rules:
+------------------------------------------------------------------+


1. Networks must be contiguous
2. Number of networks must be power of 2 (2, 4, 8, 16...)
3. First network must be divisible by total networks


Valid Supernets:
+------------------------------------------------------------------+


| Original    | Count | Summarized    |
|-------------|-------|---------------|
| .0 - .3    | 4 /24s| .0/22        |
| .0 - .7    | 8 /24s| .0/21        |
| .0 - .15   | 16 /24s| .0/20       |
| .0 - .31   | 32 /24s| .0/19       |
| .0 - .63   | 64 /24s| .0/18       |
| .0 - .127  | 128 /24s| .0/17      |


+------------------------------------------------------------------+

4.7 IPv6 Subnetting

Section titled “4.7 IPv6 Subnetting”

IPv6 uses a completely different approach.

+------------------------------------------------------------------+
|                   IPv6 Subnetting                                 |
+------------------------------------------------------------------+


IPv6 Address Structure:
+------------------------------------------------------------------+


2001:0db8:abcd:0012:0000:0000:0000:0001/64
|----------Network---------|-----Interface ID-----|


Format: 8 groups of 4 hex digits
Shortcuts:
- Leading zeros can be omitted: 2001:db8:abcd:12::1/64
- Double colon (::) for consecutive zeros (only once!)


IPv6 Prefix Lengths:
+------------------------------------------------------------------+


| /64   | Standard LAN (hosts: 2^64 - hosts are huge)              |
| /56   | Small site (256 /64s)                                    |
| /48   | Site prefix (typical allocation)                         |
| /32   | ISP allocation                                           |
| /16   | Regional registry                                        |


IPv6 Subnetting (Simple!):
+------------------------------------------------------------------+


/64 to /128: Like IPv4 /32 to /24


| Prefix   | Host bits | Hosts per subnet          |
|----------|-----------|---------------------------|
| /64      | 64 bits   | 18,446,744,073,709,551,615|
| /96      | 32 bits   | 4,294,967,295              |
| /112     | 16 bits   | 65,535                    |
| /120     | 8 bits    | 255                       |
| /128     | 0 bits    | 1 (single host)           |


Common /64 Subnet Breakdown:
+------------------------------------------------------------------+


2001:db8:abcd:0000::/64
2001:db8:abcd:0001::/64
2001:db8:abcd:0002::/64
...
2001:db8:abcd:ffff::/64


Each /64 gives 18 quintillion+ addresses!


IPv6 Subnet Calculator Logic:
+------------------------------------------------------------------+


Take /48 (site prefix), add 16 bits for subnet:


2001:db8:0000::/48 (allocated to organization)


Create /64 subnets:
2001:db8:0000::/64  - First subnet
2001:db8:0001::/64  - Second subnet
2001:db8:0002::/64  - Third subnet
...
2001:db8:ffff::/64  - 65,536 subnets!


+------------------------------------------------------------------+

4.8 Practical Subnetting Formulas

Section titled “4.8 Practical Subnetting Formulas”
+------------------------------------------------------------------+
|              Essential Subnetting Formulas                        |
+------------------------------------------------------------------+


Formula 1: Number of Hosts
+------------------------------------------------------------------+


Hosts = 2^(host bits) - 2


Example: /26
Host bits = 32 - 26 = 6
Hosts = 2^6 - 2 = 64 - 2 = 62


Formula 2: Number of Subnets
+------------------------------------------------------------------+


Subnets = 2^(borrowed bits)


Example: /24 to /26
Borrowed bits = 2
Subnets = 2^2 = 4


Formula 3: Block Size
+------------------------------------------------------------------+


Block size = 256 - (last non-zero octet of mask)


/24: 256 - 0 = 256
/25: 256 - 128 = 128
/26: 256 - 192 = 64
/27: 256 - 224 = 32
/28: 256 - 240 = 16
/29: 256 - 248 = 8
/30: 256 - 252 = 4
/31: 256 - 254 = 2
/32: 256 - 255 = 1


Formula 4: Find Nth Subnet
+------------------------------------------------------------------+


Nth subnet = (N-1) × block_size


Example: Find 3rd /26 subnet of 192.168.1.0/24
Block size = 64
3rd subnet = (3-1) × 64 = 128


Answer: 192.168.1.128/26


Formula 5: Find Subnet from IP
+------------------------------------------------------------------+


Subnet = IP AND Mask


Example: IP 192.168.1.100 with /26 (mask 255.255.255.192)
192.168.1.100 = 11000000.10101000.00000001.01100100
255.255.255.192 = 11111111.11111111.11111111.11000000
AND = 11000000.10101000.00000001.01000000
= 192.168.1.64


Subnet: 192.168.1.64/26


+------------------------------------------------------------------+

4.9 Common Subnetting Interview Questions

Section titled “4.9 Common Subnetting Interview Questions”

Question 1: How many hosts in a /23?

Section titled “Question 1: How many hosts in a /23?”
Answer:
Host bits = 32 - 23 = 9
Hosts = 2^9 - 2 = 512 - 2 = 510


Additional:
Range: 256 addresses (block size)
First 2 and last 2 are reserved
Usable: 510 hosts

Question 2: What is the network address of 172.16.45.139/25?

Section titled “Question 2: What is the network address of 172.16.45.139/25?”
Answer:
/25 = 255.255.255.128
Block size = 128


172.16.45.139 ÷ 128 = 1.xx
So it's in the 2nd /25 block


Block 1: 172.16.45.0 - 127
Block 2: 172.16.45.128 - 255


Network: 172.16.45.128
Broadcast: 172.16.45.255
Usable: 172.16.45.129 - 172.16.45.254

Question 3: Maximum hosts in 192.168.1.0/27?

Section titled “Question 3: Maximum hosts in 192.168.1.0/27?”
Answer:
/27 = /24 + 3 borrowed
Host bits = 32 - 27 = 5
Hosts = 2^5 - 2 = 32 - 2 = 30

Question 4: Subnet 192.168.10.0/24 into minimum subnets for 50 hosts each

Section titled “Question 4: Subnet 192.168.10.0/24 into minimum subnets for 50 hosts each”
Answer:
Need 50 hosts + 2 = 52 addresses
Find smallest power of 2 >= 52 = 64 = /26


/24 = 256 addresses = 4 × /26 subnets


Each /26 = 64 addresses, 62 usable = 62 > 50 ✓


4 subnets created:
192.168.10.0/26   - .0 - .63 (62 hosts)
192.168.10.64/26  - .64 - .127 (62 hosts)
192.168.10.128/26 - .128 - .191 (62 hosts)
192.168.10.192/26 - .192 - .255 (62 hosts)

Question 5: What is the CIDR notation for 255.255.255.240?

Section titled “Question 5: What is the CIDR notation for 255.255.255.240?”
Answer:
255.255.255.240 = 11111111.11111111.11111111.11110000


Count the 1s: 8 + 8 + 8 + 4 = 28


CIDR: /28


Hosts: 2^(32-28) - 2 = 2^4 - 2 = 16 - 2 = 14

4.10 Chapter Summary

Section titled “4.10 Chapter Summary”

In this comprehensive chapter, you learned:

  • ✅ Why subnetting exists - address exhaustion, performance, security
  • ✅ Binary math - conversions, powers of 2
  • ✅ IP address classes - A, B, C, D, E and special addresses
  • ✅ Subnet masks - all common masks, CIDR notation
  • ✅ Step-by-step subnetting - multiple detailed examples
  • ✅ VLSM - Variable Length Subnet Masking
  • ✅ Supernetting - route summarization
  • ✅ IPv6 subnetting - completely different approach
  • ✅ Formulas - quick reference for all calculations
  • ✅ Interview questions - with detailed answers

Master these concepts and subnetting will become second nature.

Next Chapter

Section titled “Next Chapter”

Chapter 5: DNS - Domain Name System

Last Updated: February 2026