13. Subnetting_Part1

IP Address Classes and Prefix Lengths

In IPv4, addresses are categorized into classes, but only Class A, B, and C addresses can be assigned to devices as IP addresses:

• Class A: Prefix Length /8
• Class B: Prefix Length /16
• Class C: Prefix Length /24

IP Address Allocation and Wastefulness

The Internet Assigned Numbers Authority (IANA) assigns IPv4 addresses/networks to companies based on their size. However, the “classful” system led to significant waste of IP addresses. For example, a company needing 5,000 addresses would be assigned a Class B IP address, which has over 65,000 addresses—resulting in more than 60,000 unused addresses.

Introduction of CIDR

To address this inefficiency, the Internet Engineering Task Force (IETF) introduced CIDR (Classless Inter-Domain Routing) in 1993. CIDR removes the rigid size constraints of Class A, B, and C networks, allowing them to be divided into smaller, more efficient networks called “subnets.”

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Calculating Usable IP Addresses

To determine how many usable IP addresses are in a given network:

$\text{Usable Addresses}=2^n-2$

Where:

• n = Number of host bits

Example: CIDR Practice with /25

• Network: 203.0.113.0/25

• Binary Representation:

  1100 1011 . 0000 0000 . 0111 0001 . 0 | 000 0000

• Subnet Mask:

  1111 1111 . 1111 1111 . 1111 1111 . 1 | 000 0000

  This equals 255.255.255.128.

• Usable Hosts:

  2^(7 bits) - 2 = 126 hosts

Example: CIDR Practice with /28

• Network: 203.0.113.0/28

• Binary Representation:

  1100 1011 . 0000 0000 . 0111 0001 . 0000 | 0000

• Subnet Mask:

  1111 1111 . 1111 1111 . 1111 1111 . 1111 | 0000

  This equals 255.255.255.240.

• Usable Hosts:

  2^(4 bits) - 2 = 14 hosts

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Subnetting Cheatsheet

Group Size	128	64	32	16	8	4	2	1
Subnet Mask	128	192	224	240	248	252	254	255
CIDR	/25	/26	/27	/28	/29	/30	/31	/32
3rd Octet	/17	/18	/19	/20	/21	/22	/23	/24
2nd Octet	/9	/10	/11	/12	/13	/14	/15	/16
1st Octet	/1	/2	/3	/4	/5	/6	/7	/8

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Subnetting Process Overview

1. Identify CIDR/Subnet Mask: Locate it on the cheat sheet.
2. Find Group Size: Increase by group size until passing the target IP.
3. Calculate:
   • Network ID: Number before the target IP.
   • Next Network: Number after the target IP.
   • Broadcast Address: IP address before the next network.
   • First Host: IP address after the network ID.
   • Last Host: IP address before the broadcast IP.
   • Total Addresses: Group size (minus 2 for usable addresses).

Example: Solving CIDR for 3rd Octet IPs

• Address: 10.4.77.188/19
• Subnet Mask: 255.255.224.0
• Group Size: 256 - 224 = 32
• Range: 10.4.64.0 to 10.4.95.255
• Usable Addresses: 8192

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Alternative Subnetting Method (Without Cheat Sheet)

1. Identify the “Magic Octet”: Where the IP/Prefix lies.
2. Count Network Bits: Determine the address group size.
3. Calculate Subnet Mask: Based on the magic octet.
4. Determine Base Network and Broadcast:
   • If remainder in division, use integer times group size.
   • No remainder, base address is that octet value.
5. Calculate Subnets and Usable Hosts.

Example 1:

• Address: 154.219.154.180/20
• Magic Octet: 3rd
• Subnet Mask: 255.255.240.0
• Network: 154.219.144.0
• Broadcast: 154.219.159.255
• Subnets: 16
• Total Hosts: 4094

Example 2:

• Address: 84.75.21.6/10
• Magic Octet: 2nd
• Subnet Mask: 255.192.0.0
• Network: 84.64.0.0
• Broadcast: 84.127.255.255
• Subnets: 4
• Total Hosts: 4,194,302

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