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Lecture 12 - IP Addressing and Subnets

Updated 4 Oct 2026

#FinalExam very important

IP Addressing Basics

  • IP Addressing is Logical Addressing
    • Works on Network Layer (Layer 3)
  • Two versions of Addressing Scheme:
    • IPv4 – 32 bit addressing
    • IPv6 – 128 bit addressing

ลองเช็คดูได้

ifconfig

IPv4

  • Bit representation: A value that represents 0's or 1's (Binary)
    • Example: 01010101000001011011111100000001
  • 32 bits divided into 4 Octets (Dotted Decimal Notation)
    • Format: 01010101.00000101.10111111.00000001
    • Structure: First Octet | Second Octet | Third Octet | Fourth Octet

Think of IPv4 like a postal address with 4 sections, where each section can range from 0 to 255

IPv6

  • 128-bit address divided along 16-bit boundaries
  • Each 16-bit block converted to 4-digit hexadecimal number
  • Separated by colons (Colon-Hex Notation)
  • Example: FEDC:BA98:7654:3210:FEDC:BA98:7654:3210

Binary to Decimal Conversion

Example for First Octet:

  • Total 8 bits with values 0's and 1's
  • 28=2562^8 = 256 combinations
272^7262^6252^5242^4232^3222^2212^1202^0Decimal
000000000
000000011
000000102
000000113
000001004
...........................
11111111255
Total IP Address Range: 0.0.0.0 to 255.255.255.255

IP Address Classes

The total IP addressing scheme is divided into 5 Classes:

  1. CLASS A – LAN & WAN
  2. CLASS B – LAN & WAN
  3. CLASS C – LAN & WAN
  4. CLASS D – Multicasting
  5. CLASS E – Research & Development

Priority Bit Concept

To identify the range of each class, a priority bit is used (leftmost bits in the first octet):

  • CLASS A: Priority bit is 0
  • CLASS B: Priority bit is 10
  • CLASS C: Priority bit is 110
  • CLASS D: Priority bit is 1110
  • CLASS E: Priority bit is 1111

Priority bits are like area codes - they tell you which "region" (class) an IP address belongs to

แต่ละ Class ดูยังไง ก็จะมี Priority bit นั่นก็คือ Leftmost bit in the first octet

CLASS A Range

Format: 0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

First bit of first octet reserved for priority bit:

272^7262^6252^5242^4232^3222^2212^1202^0Decimal
000000000
000000011
...........................
01111111127

Class A Range: 0.0.0.0 to 127.255.255.255\boxed{\text{Class A Range: } 0.0.0.0 \text{ to } 127.255.255.255}

CLASS B Range

Format: 10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

First two bits of first octet reserved for priority bit:

272^7262^6252^5242^4232^3222^2212^1202^0Decimal
10000000128
10000001129
...........................
10111111191

Class B Range: 128.0.0.0 to 191.255.255.255\boxed{\text{Class B Range: } 128.0.0.0 \text{ to } 191.255.255.255}

CLASS C Range

Format: 110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

First three bits of first octet reserved for priority bit:

272^7262^6252^5242^4232^3222^2212^1202^0Decimal
11000000192
11000001193
...........................
11011111223

Class C Range: 192.0.0.0 to 223.255.255.255\boxed{\text{Class C Range: } 192.0.0.0 \text{ to } 223.255.255.255}

จากที่เราคุ้นเคยกันเลยล่ะ 192.168.1.1

CLASS D Range

Format: 1110xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

First four bits of first octet reserved for priority bit:

272^7262^6252^5242^4232^3222^2212^1202^0Decimal
11100000224
11100001225
...........................
11101111239

Class D Range: 224.0.0.0 to 239.255.255.255\boxed{\text{Class D Range: } 224.0.0.0 \text{ to } 239.255.255.255}

CLASS E Range

Format: 1111xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

First four bits of first octet reserved for priority bit:

272^7262^6252^5242^4232^3222^2212^1202^0Decimal
11110000240
11110001241
...........................
11111111255

Class E Range: 240.0.0.0 to 255.255.255.255\boxed{\text{Class E Range: } 240.0.0.0 \text{ to } 255.255.255.255}

Octet Format

IP address is divided into Network & Host Portion:

  • CLASS A: N.H.H.H (Network.Host.Host.Host)
  • CLASS B: N.N.H.H (Network.Network.Host.Host)
  • CLASS C: N.N.N.H (Network.Network.Network.Host)

Think of N (Network) as the street name and H (Host) as the house number

CLASS A – Number of Networks & Hosts

Octet Format: N.H.H.H

  • Network bits: 8
  • Host bits: 24

Number of Networks:
Networks=28−1−2=27−2=128−2=126 Networks\text{Networks} = 2^{8-1} - 2 = 2^7 - 2 = 128 - 2 = \boxed{126 \text{ Networks}}

  • (-1 is for Priority Bit)
  • (-2 is for network 0 and network 127) — เพราะว่ามัน Reserved!

Number of Hosts:
Hosts=224−2=16,777,216−2=16,777,214 Hosts/Network\text{Hosts} = 2^{24} - 2 = 16,777,216 - 2 = \boxed{16,777,214 \text{ Hosts/Network}}

  • (-2 is for Network ID & Broadcast ID)

เยอะมาก พวกนี้ก็เป็นของ Service Provider (ISP) ทั้งหลาย ไม่ใช่ชาวบ้านแบบเรา ๆ

CLASS B – Number of Networks & Hosts

Octet Format: N.N.H.H

  • Network bits: 16
  • Host bits: 16

Number of Networks:
Networks=216−2−2=214−2=16,384−2=16,382 Networks\text{Networks} = 2^{16-2} - 2 = 2^{14} - 2 = 16,384 - 2 = \boxed{16,382 \text{ Networks}}

  • (-2 is for Priority Bits)
  • (-2 is for reserved networks)

Number of Hosts:
Hosts=216−2=65,536−2=65,534 Hosts/Network\text{Hosts} = 2^{16} - 2 = 65,536 - 2 = \boxed{65,534 \text{ Hosts/Network}}

  • (-2 is for Network ID & Broadcast ID)

Middle-size network

CLASS C – Number of Networks & Hosts

Octet Format: N.N.N.H

  • Network bits: 24
  • Host bits: 8

Number of Networks:
Networks=224−3−2=221−2=2,097,152−2=2,097,150 Networks\text{Networks} = 2^{24-3} - 2 = 2^{21} - 2 = 2,097,152 - 2 = \boxed{2,097,150 \text{ Networks}}

  • (-3 is for Priority Bits)
  • (-2 is for reserved networks)

Number of Hosts:
Hosts=28−2=256−2=254 Hosts/Network\text{Hosts} = 2^{8} - 2 = 256 - 2 = \boxed{254 \text{ Hosts/Network}}

  • (-2 is for Network ID & Broadcast ID)

Network & Broadcast Address

Key Concepts:

มาละ ตรงที่บอกว่าถูก Reserved อะ มันคือตรงนี้นั่นเองงง

  • Network Address: All bits are ZERO in the host portion
  • Broadcast Address: All bits are ONES in the host portion
  • Valid IP Addresses: Lie between Network Address and Broadcast Address
  • Only Valid IP Addresses are assigned to hosts/clients

Network address = "start of the street", Broadcast address = "end of the street", Valid IPs = "houses on the street"

Class A Example: Network 10.0.0.0

Format: N.H.H.H

  • Network Address: 0xxxxxxx.00000000.00000000.00000000
  • Broadcast Address: 0xxxxxxx.11111111.11111111.11111111

Example:

  • Network Address: 10.0.0.0
  • Valid IP Addresses: 10.0.0.1 to 10.255.255.254
  • Broadcast Address: 10.255.255.255

Class B Example: Network 172.16.0.0

Format: N.N.H.H

  • Network Address: 10xxxxxx.xxxxxxxx.00000000.00000000
  • Broadcast Address: 10xxxxxx.xxxxxxxx.11111111.11111111

Example:

  • Network Address: 172.16.0.0
  • Valid IP Addresses: 172.16.0.1 to 172.16.255.254
  • Broadcast Address: 172.16.255.255

Class C Example: Network 192.168.1.0

Format: N.N.N.H

  • Network Address: 110xxxxx.xxxxxxxx.xxxxxxxx.00000000
  • Broadcast Address: 110xxxxx.xxxxxxxx.xxxxxxxx.11111111

Example:

  • Network Address: 192.168.1.0
  • Valid IP Addresses: 192.168.1.1 to 192.168.1.254
  • Broadcast Address: 192.168.1.255

Private IP Addresses

Certain addresses in each class are reserved for LAN (Local Area Network):

  • Used for home & office networks
  • Networks not connected to Internet
\begin{align} \text{Class A: } &10.0.0.0 \text{ to } 10.255.255.255 \\ \text{Class B: } &172.16.0.0 \text{ to } 172.31.255.255 \\ \text{Class C: } &192.168.0.0 \text{ to } 192.168.255.255 \end{align} }$$ > Private IPs are like internal phone extensions - they work within your building but can't be dialed from outside >คือ Address พวกนี้มันจะไม่สามารถต่อตรงกับ Internet ได้นะ ถึงว่าวงในบ้านส่วนใหญ่มันเริ่มต้นด้วย 192.168.x.x ใช่มะ → แล้วจะต่อกับ Internet ยังไง? → ต้องทำบน Router NAT คอยแปลง Private IP เป็น Public IP ที่ ISP ให้มาชั่วคราว นี่เป็นการยืด IPv4 ให้ยังใช้ได้ถึงทุกวันนี้แบบไม่ซ้ำ ___ ## Subnet Mask >แล้ว Computer รู้ได้ไงว่า อย่าง IP ที่มันได้มาเช่น 192.168.1.100 ส่วนไหนคือ Network ส่วนไหนคือ Host? >ก็ต้องมีตัวช่วยก็คือ **Subnet Mask** **Purpose:** - Differentiates Network portion and Host Portion - Presented as all 1's in network portion and all 0's in host portion >แต่เครื่องคอมพิวเตอร์ไม่รู้หรอกว่าจะแยกตรงไหน ถ้าไม่มี “หน้ากาก” เลยต้องมี **Subnet Mask** เพื่อ “บอกขอบเขต” ว่าส่วนไหนคือ Network และส่วนไหนคือ Host ### Default Subnet Masks **Class A:** N.H.H.H ``` 11111111.00000000.00000000.00000000 ``` $$\boxed{\text{Default Subnet Mask: } 255.0.0.0}$$ **Class B:** N.N.H.H ``` 11111111.11111111.00000000.00000000 ``` $$\boxed{\text{Default Subnet Mask: } 255.255.0.0}$$ **Class C:** N.N.N.H ``` 11111111.11111111.11111111.00000000 ``` $$\boxed{\text{Default Subnet Mask: } 255.255.255.0}$$ ## How Subnet Mask Works (AND Operation) **Example:** - IP Address: `192.168.1.1` - Subnet Mask: `255.255.255.0` **ANDING PROCESS:** ``` 192.168.1.1 = 11000000.10101000.00000001.00000001 255.255.255.0 = 11111111.11111111.11111111.00000000 ========================================= 192.168.1.0 = 11000000.10101000.00000001.00000000 ``` >Network address (`192.168.1.0`) คือ “ชื่อเครือข่าย” ที่เครื่องนี้อยู่ในนั้น **AND Truth Table:** | A | B | C | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 1 | > The output is 1 only if both inputs are 1, otherwise it's 0 ## Subnetting >แก้ปัญหาความไม่ยืดหยุ่นของ Class → กระบวนการที่แบ่งเครือข่ายใหญ่ ๆ ที่ได้มาตาม Class เอามาแบ่งเป็นเครือข่ายย่อย ๆ ตามที่ต้องการ **หลักการหลัก ๆ ก็คือการยืม Bit จากส่วนที่เคยเป็น Host มาเป็นส่วนของ Subnet แทน** **Definition:** - Process of ==breaking large network into small networks (subnets)== - Extends default boundary of subnet mask - Borrows host bits to create networks (dividing a single network into multiple networks) - Converting host bits to network bits (changing 0's to 1's) - Also called **FLSM (Fixed Length Subnet Mask)** **Three Ways to Subnet:** 1. Requirement of Networks 2. Requirement of Hosts 3. Cisco / Notation > Subnetting is like dividing a large apartment building into separate floors, each with its own address range ### Subnetting Scenario Example **Given:** 100 PCs in 5 departments **Which Class?** Class C (sufficient for 254 hosts) #### Solution 1 (Poor): ``` - Sales: 192.168.1.1 to 192.168.1.20 - Accounting: 192.168.1.21 to 192.168.1.40 - Engineering: 192.168.1.41 to 192.168.1.60 - Management: 192.168.1.61 to 192.168.1.80 - Training: 192.168.1.81 to 192.168.1.100 ``` **Problems:** - Difficult to distinguish IP addresses for each department - All use same network mask (255.255.255.0) #### Solution 2 (Poor): ``` - Sales: 192.168.1.1 to 192.168.1.20 - Accounting: 192.168.2.1 to 192.168.2.20 - Engineering: 192.168.3.1 to 192.168.3.20 - Management: 192.168.4.1 to 192.168.4.20 - Training: 192.168.5.1 to 192.168.5.20 ``` **Problems:** - Loss of bandwidth (broadcasting for 254 machines instead of 20) - Wastage of IP addresses (~1000 wasted รวมทั้ง 5 domain ละ) - No security separation **Best Solution:** Use Subnetting! ✓ ### Subnetting Example 1 - **Requirement:** 5 Networks needed - **Given:** Class C: `192.168.1.0` - **Format:** `110xxxxx.xxxxxxxx.xxxxxxxx.xxx|xxxxx` >Borrow from the host part สมมติขอมา 3 bits #### Calculations: **Number of Subnets:** $$\text{Subnets} = 2^n - 2 \geq 5$$ $$2^3 - 2 = 8 - 2 = \boxed{6 \text{ Subnets}}$$ - We borrow 3 bits for subnetting - (-2 is for First & Last Subnet Range) **Number of Hosts:** $$\text{Hosts} = 2^h - 2$$ $$\text{where } h = 8 - 3 = 5 \text{ bits remaining}$$ $$2^5 - 2 = 32 - 2 = \boxed{30 \text{ Hosts/Subnet}}$$ - (-2 is for Network ID & Broadcast ID) >ก็คือเราสามารถ Support ได้ 30 PCs/Department อะไรทำนองนั้น #### Custom Subnet Mask: ``` 255.255.255.11100000 ``` $$\boxed{\text{Custom Subnet Mask: } 255.255.255.224}$$ #### Subnet Ranges: | Network ID | Valid Host Range | Broadcast ID | Status | | ------------- | ---------------- | ------------- | --------- | | 192.168.1.0 | — | 192.168.1.31 | Reserved | | 192.168.1.32 | .33 to .62 | 192.168.1.63 | **Valid** | | 192.168.1.64 | .65 to .94 | 192.168.1.95 | **Valid** | | 192.168.1.96 | .97 to .126 | 192.168.1.127 | **Valid** | | 192.168.1.128 | .129 to .158 | 192.168.1.159 | **Valid** | | 192.168.1.160 | .161 to .190 | 192.168.1.191 | **Valid** | | 192.168.1.192 | .193 to .222 | 192.168.1.223 | **Valid** | | 192.168.1.224 | — | 192.168.1.255 | Reserved | #### Department Allocation: ``` - Sales: 192.168.1.32 to 192.168.1.63 (30 hosts) - Accounting: 192.168.1.64 to 192.168.1.95 (30 hosts) - Engineering: 192.168.1.96 to 192.168.1.127 (30 hosts) - Management: 192.168.1.128 to 192.168.1.159 (30 hosts) - Training: 192.168.1.160 to 192.168.1.191 (30 hosts) - Future Use: 192.168.1.192 to 192.168.1.223 (30 hosts) ``` ### Subnetting Example 2 - **Requirement:** 14 Networks needed - **Given:** Class C: `192.168.1.0` - **Format:** `110xxxxx.xxxxxxxx.xxxxxxxx.xxxx|xxxx` #### Calculations: **Number of Subnets:** $$\text{Subnets} = 2^n - 2 \geq 14$$ $$2^4 - 2 = 16 - 2 = \boxed{14 \text{ Subnets}}$$ - We borrow 4 bits for subnetting - (-2 is for First & Last Subnet Range) **Number of Hosts:** $$\text{Hosts} = 2^h - 2$$ $$\text{where } h = 8 - 4 = 4 \text{ bits remaining}$$ $$2^4 - 2 = 16 - 2 = \boxed{14 \text{ Hosts/Subnet}}$$ #### Custom Subnet Mask: ``` 255.255.255.11110000 ``` $$\boxed{\text{Custom Subnet Mask: } 255.255.255.240}$$ #### Sample Subnet Ranges: | Network ID | Valid Host Range | Broadcast ID | Status | |-----------------|---------------------|-----------------|-------------| | 192.168.1.0 | — | 192.168.1.15 | Reserved | | 192.168.1.16 | .17 to .30 | 192.168.1.31 | **Valid** | | 192.168.1.32 | .33 to .46 | 192.168.1.47 | **Valid** | | 192.168.1.48 | .49 to .62 | 192.168.1.63 | **Valid** | | 192.168.1.64 | .65 to .79 | 192.168.1.80 | **Valid** | | ... | ... | ... | ... | | 192.168.1.224 | .225 to .238 | 192.168.1.239 | **Valid** | | 192.168.1.240 | — | 192.168.1.255 | Reserved | ### Important Note on Subnetting > [!important] >**The number of subnets must be a power of 2** **Valid subnet counts:** 2, 4, 8, 16, 32, 64, 128, 256... > IP addresses are designed with **two levels of hierarchy**: > 1. Network portion > 2. Host portion > With subnetting, we create a **three-level hierarchy**: > 1. Network prefix > 2. Subnet number > 3. Host number ## Basic Idea of Subnetting **Process:** - Split the host number portion of an IP address into: - Subnet number - Smaller host number **Result:** 3-layer hierarchy ![[Pasted image 20251016094703.png|center|500]] **Benefits:** - Subnets can be freely assigned within the organization - Internally, subnets are treated as separate networks - Subnet structure is **not visible outside** the organization ## Subnet Example (Class B) **Network Address:** `172.16.0.0` with `/16` network mask ### Without Subnetting: ``` 172.16.0.Host (Network | Network | Host) 172.16.1.Host 172.16.2.Host ... 172.16.255.Host ``` ### Using Subnets: `/24` or `255.255.255.0` >**คือเขียนแบบนี้ก็ได้ /24 (คือจำนวน Bit ของ Network + รวมกับที่ยืมมาด้วย**) **Structure:** Network | Network | Subnet | Host ![[Pasted image 20251016095035.png|center|500]] **Calculations:** - **Number of subnets:** $2^8 - 2 = 254$ valid subnets - First subnet usually reserved - Last subnet reserved for broadcast - **Hosts per subnet:** $2^8 - 2 = 254$ hosts - Each subnet has 254 usable host addresses ![[Pasted image 20251016095118.png|center|500]] **Broadcast Addresses:** ![[Pasted image 20251016095100.png|center|500]] --- ## Advantages of Subnetting 1. **3-Layer Hierarchy:** - Network - Subnet - Host 2. **Reduces Router Complexity:** - External routers don't know about subnetting - Complexity of routing tables at external routers is reduced - Simplifies network management 3. **Better Organization:** - Logical separation of departments/functions - Improved security - Efficient use of IP address space --- ## Default Mask vs Subnet Mask ![[Pasted image 20251016095230.png|center|400]] ### Without Subnetting (Default Mask) **Example:** IP `141.14.72.24` with default mask `255.255.0.0` ``` 141.14.72.24 = 11000000 00101101 01001000 00011000 255.255.0.0 = 11111111 11111111 00000000 00000000 ========================================= 141.14.0.0 = 11000000 00101101 00000000 00000000 ``` **Result:** Network Address = `141.14.0.0` ### With Subnetting (Subnet Mask) **Example:** IP `141.14.72.24` with subnet mask `255.255.192.0` ``` 141.14.72.24 = 11000000 00101101 01001000 00011000 255.255.192.0 = 11111111 11111111 11000000 00000000 ========================================= 141.14.64.0 = 11000000 00101101 01000000 00000000 ``` **Result:** Subnet Address = `141.14.64.0` > The subnet mask creates a more specific network address by using additional bits --- ## Finding the Subnet Address - Two methods to find subnet address: 1. **Straight Method:** Binary AND operation 2. **Short-Cut Method:** Selective copying ### Straight Method Use binary notation for both address and mask, then apply AND operation. **Example:** Find subnet address for `200.45.34.56` with mask `255.255.240.0` (/20) ``` 200.45.34.56 = 11001000 00101101 00100010 00111000 255.255.240.0 = 11111111 11111111 11110000 00000000 ========================================= 200.45.32.0 = 11001000 00101101 00100000 00000000 ``` $$\boxed{\text{Subnet Address: } 200.45.32.0}$$ ### Short-Cut Method **Rules:** 1. If byte in mask is **255** → Copy the byte in the address 2. If byte in mask is **0** → Replace the byte in address with 0 3. If byte in mask is **neither 255 nor 0** → Use binary AND for that byte only **Example:** Find subnet for `19.30.84.5` with mask `255.255.192.0` (/24) ``` IP: 19 . 30 . 84 . 5 Mask: 255 . 255 . 192 . 0 ↓ ↓ ↓ ↓ 19 . 30 . ? . 0 ``` ![[Pasted image 20251016095422.png|center|500]] For the third byte (84 AND 192): ``` 84 = 01010100 192 = 11000000 -------- 64 = 01000000 ``` $$\boxed{\text{Subnet Address: } 19.30.64.0}$$ ## Comparison: Default Mask vs Subnet Mask ### Class B Example **Default Mask:** `255.255.0.0` ``` 11111111 11111111 | 00000000 00000000 ←------ 16 -------→ Network bits ``` **Subnet Mask:** `255.255.224.0` ``` 11111111 11111111 111 | 00000 00000000 ←-------- 19 --------→ ←---- 13 ---→ Extended Network bits Host bits ↑___↑ 3 subnet bits ``` > By borrowing 3 bits from the host portion, we can create $2^3 = 8$ subnets --- ## Subnetting Practice Example >ค่อยแก้นะ **Given:** - **Host IP Address:** `172.16.18.33` - **Network Mask:** `255.255.0.0` - **Subnet Mask:** `255.255.255.0` ### Find: #### Major Network Information: - **Major Network Address:** `172.16.0.0` - **Major Network Broadcast:** `172.16.255.255` - **Range if not subnetted:** `172.16.0.1` to `172.16.255.254` #### Subnet Information: - **Subnet Address:** `172.16.18.0` - **First Host:** `172.16.18.1` - **Last Host:** `172.16.18.254` - **Broadcast Address:** `172.16.18.255` #### Other Subnet Information: - **Total subnets:** $2^8 - 2 = 254$ subnets - **Hosts per subnet:** $2^8 - 2 = 254$ hosts --- ## In-class Exercise 1: Class B Subnetting **Given:** - Site address: `181.56.0.0` (Class B) - Requirement: 1000 subnets ### Solution: **Step 1:** Determine bits needed for subnets - Default mask has **16 ones** (Class B) - Need 1000 subnets - 1000 is not a power of 2 - Next power of 2: $2^{10} = 1024$ ≥ 1000 - Need to borrow **10 bits** from host portion **Step 2:** Calculate subnet mask - Total 1s in subnet mask: $16 + 10 = 26$ - Total 0s: $32 - 26 = 6$ **Subnet Mask:** ``` 11111111.11111111.11111111.11000000 ``` $\boxed{\text{Subnet Mask: } 255.255.255.192 \text{ or } /26}$ **Step 3:** Calculate results - **Number of subnets:** $2^{10} = 1024$ subnets - **Hosts per subnet:** $2^6 - 2 = 64 - 2 = 62$ hosts --- ## In-class Exercise 2: Finding Subnet Information **Given:** - **Host IP Address:** `138.101.114.250` - **Network Mask:** `255.255.0.0` - **Subnet Mask:** `255.255.255.192` ### Major Network Information: - **Major Network Address:** `138.101.0.0` - **Major Network Broadcast:** `138.101.255.255` (Last address) - **Range if not subnetted:** `138.101.0.1` to `138.101.255.254` ### Finding Subnet Address: Using the short-cut method: ``` IP: 138 . 101 . 114 . 250 Mask: 255 . 255 . 255 . 192 ↓ ↓ ↓ ↓ 138 . 101 . 114 . ? ``` For the fourth byte (250 AND 192): ``` 250 = 11111010 192 = 11000000 -------- 192 = 11000000 ``` **Subnet Address:** `138.101.114.192` ### Subnet Information: - **Subnet Address:** `138.101.114.192` - **First Host:** `138.101.114.193` - **Last Host:** `138.101.114.254` - **Broadcast Address:** `138.101.114.255` ### Other Subnet Information: **Bits borrowed:** 10 bits (subnet mask has 26 ones, default has 16) - **Total subnets:** $2^{10} = 1024$ subnets - **Hosts per subnet:** $2^6 - 2 = 62$ hosts --- ## Variable-Length Subnetting (VLSM) >อนุญาตให้เราใช้ Subnet Mask ที่มันขนาดแตกต่างกันได้ ภายในเครือข่ายหลักเดียวกัน VLSM allows different subnet masks within the same network to optimize IP address usage. ![[Pasted image 20251016095715.png|center|500]] ### Example Scenario: **Site:** `X.Y.Z.0` **Requirements:** - 3 subnets with 62 hosts each - 2 subnets with 30 hosts each ### Solution: **First Level Subnetting:** - **First Mask:** `255.255.255.192` (/26) - Creates subnets with 62 usable hosts - For the three 62-host networks **Second Level Subnetting:** - **Second Mask:** `255.255.255.224` (/27) - Creates subnets with 30 usable hosts - Further subdivide one /26 subnet into two /27 subnets > VLSM is like having different-sized rooms in a building - you use the space more efficiently by matching room size to actual needs ### Benefits: - Efficient IP address utilization - Flexibility in network design - Reduced IP address wastage - Better scalability --- ## Appendix: Reference Tables ### Power Table | Power | Value | Power | Value | |-------|--------------|-------|--------------| | $2^1$ | 2 | $2^{17}$ | 131,072 | | $2^2$ | 4 | $2^{18}$ | 262,144 | | $2^3$ | 8 | $2^{19}$ | 524,288 | | $2^4$ | 16 | $2^{20}$ | 1,048,576 | | $2^5$ | 32 | $2^{21}$ | 2,097,152 | | $2^6$ | 64 | $2^{22}$ | 4,194,304 | | $2^7$ | 128 | $2^{23}$ | 8,388,608 | | $2^8$ | 256 | $2^{24}$ | 16,777,216 | | $2^9$ | 512 | $2^{25}$ | 33,554,432 | | $2^{10}$ | 1,024 | $2^{26}$ | 67,108,864 | | $2^{11}$ | 2,048 | $2^{27}$ | 134,217,728 | | $2^{12}$ | 4,096 | $2^{28}$ | 268,435,456 | | $2^{13}$ | 8,192 | $2^{29}$ | 536,870,912 | | $2^{14}$ | 16,384 | $2^{30}$ | 1,073,741,824 | | $2^{15}$ | 32,768 | $2^{31}$ | 2,147,483,648 | | $2^{16}$ | 65,536 | $2^{32}$ | 4,294,967,296 | --- ### Important Subnet Mask Values | Bits | Binary | Decimal | CIDR | |------|-----------|---------|------| | 1 | 10000000 | 128 | /25 | | 2 | 11000000 | 192 | /26 | | 3 | 11100000 | 224 | /27 | | 4 | 11110000 | 240 | /28 | | 5 | 11111000 | 248 | /29 | | 6 | 11111100 | 252 | /30 | | 7 | 11111110 | 254 | /31 | | 8 | 11111111 | 255 | /32 | > Memorize these values! They're essential for quick subnet calculations --- ## Quick Reference: Subnetting Formulas ### Number of Subnets $\boxed{\text{Subnets} = 2^n - 2}$ where $n$ = number of borrowed bits > Some modern networks don't subtract 2, using all subnets including first and last ### Number of Hosts per Subnet $\boxed{\text{Hosts} = 2^h - 2}$ where $h$ = number of remaining host bits - Subtract 2 for Network ID and Broadcast ID ### Total Bits in Subnet Mask $\boxed{\text{Total 1s} = \text{Default mask bits} + \text{Borrowed bits}}$ ### Finding Network Increment $\boxed{\text{Increment} = 256 - \text{Subnet octet value}}$ **Example:** For mask `255.255.255.224` - Increment = $256 - 224 = 32$ - Subnets: 0, 32, 64, 96, 128, 160, 192, 224 --- ## Subnetting Strategy Guide ### Step-by-Step Process: 1. **Identify the class** (A, B, or C) from the IP address 2. **Determine requirements:** - How many subnets needed? - How many hosts per subnet? 3. **Calculate bits to borrow:** - For subnets: Find smallest $n$ where $2^n \geq$ required subnets - For hosts: Find smallest $h$ where $2^h - 2 \geq$ required hosts 4. **Create custom subnet mask:** - Add borrowed bits to default mask 5. **Calculate subnet ranges:** - Find increment value - List all subnet boundaries 6. **Assign subnets** to network segments ### Common Subnet Mask Quick Reference: | CIDR | Subnet Mask | # Hosts | Class C Subnets | |------|-------------------|---------|-----------------| | /24 | 255.255.255.0 | 254 | 1 | | /25 | 255.255.255.128 | 126 | 2 | | /26 | 255.255.255.192 | 62 | 4 | | /27 | 255.255.255.224 | 30 | 8 | | /28 | 255.255.255.240 | 14 | 16 | | /29 | 255.255.255.248 | 6 | 32 | | /30 | 255.255.255.252 | 2 | 64 | --- ## Tips and Tricks ### Quick IP Class Identification: - **Class A:** First octet 1-126 (starts with 0) - **Class B:** First octet 128-191 (starts with 10) - **Class C:** First octet 192-223 (starts with 110) - **Class D:** First octet 224-239 (starts with 1110) - Multicast - **Class E:** First octet 240-255 (starts with 1111) - Reserved ### Special Addresses to Remember: - `127.x.x.x` - Loopback (localhost) - `0.0.0.0` - Default route - `255.255.255.255` - Broadcast to all ### Binary Conversion Shortcuts: - **128** = 10000000 - **192** = 11000000 - **224** = 11100000 - **240** = 11110000 - **248** = 11111000 - **252** = 11111100 - **254** = 11111110 - **255** = 11111111 ### Common Pitfalls to Avoid: 1. ❌ Forgetting to subtract 2 for network and broadcast addresses 2. ❌ Not using power of 2 for subnet count 3. ❌ Mixing up host bits and network bits 4. ❌ Using first or last subnet (in older routing protocols) 5. ❌ Forgetting that /24, /16, /8 are the default masks --- ## Practice Problems Summary ### Problem Type 1: Given Subnet Requirements **Find:** Custom subnet mask, number of hosts **Example:** Need 6 subnets in Class C - Use 3 bits ($2^3 = 8 \geq 6$) - Mask: 255.255.255.224 - Hosts per subnet: $2^5 - 2 = 30$ ### Problem Type 2: Given IP and Masks **Find:** Network address, subnet address, broadcast address, host range **Example:** IP `172.16.18.33` with mask `255.255.255.0` - Network: 172.16.0.0 - Subnet: 172.16.18.0 - Broadcast: 172.16.18.255 - Host range: 172.16.18.1 - 172.16.18.254 ### Problem Type 3: VLSM Design **Find:** Optimal subnet allocation for varying host requirements **Strategy:** - Start with largest subnet requirement - Allocate from beginning of address space - Subdivide remaining space for smaller subnets --- ## Final Notes ### Key Takeaways: 1. ✅ Subnetting creates logical network divisions 2. ✅ Always use powers of 2 for subnet calculations 3. ✅ Remember to subtract 2 (network ID and broadcast) 4. ✅ External routers don't see internal subnetting 5. ✅ VLSM allows different mask lengths for efficiency ### When to Use Each Class: - **Class A:** Very large organizations (millions of hosts) - **Class B:** Medium to large organizations (thousands of hosts) - **Class C:** Small organizations (up to 254 hosts) ### Modern Considerations: - **CIDR** (Classless Inter-Domain Routing) is now more common than classful addressing - **IPv6** is gradually replacing IPv4 due to address exhaustion - **Private IP ranges** are essential for NAT (Network Address Translation)