CCNA-Notes/13. Subnetting (Part 1).md

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13. Subnetting (Part 1)

Subnetting (Part 1)

IPv4 Address Classes

Class	First Octet (Binary)	Range	Default Prefix
A	0xxxxxxx	0–127	/8
B	10xxxxxx	128–191	/16
C	110xxxxx	192–223	/24
D	1110xxxx	224–239	N/A (Multicast)
E	1111xxxx	240–255	N/A (Reserved)

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Maximum Hosts per Network

• Host bits all 0s → Network address
• Host bits all 1s → Broadcast address

Example Calculations

192.168.1.0/24 → 192.168.1.255

• Host bits: 8
• Total addresses: 2⁸ = 256
• Usable hosts: 256 − 2 = 254

172.16.0.0/16 → 172.16.255.255

• Host bits: 16
• Total addresses: 2¹⁶ = 65,536
• Usable hosts: 65,536 − 2 = 65,534

10.0.0.0/8 → 10.255.255.255

• Host bits: 24
• Total addresses: 2²⁴ = 16,777,216
• Usable hosts: 16,777,216 − 2 = 16,777,214

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How IP Addresses Were Distributed

• The IANA (Internet Assigned Numbers Authority) originally allocated IP ranges based on class.
• Large organizations received Class A or B networks, while smaller ones received Class C.
• This rigid system caused significant address waste.

Example 1: Point-to-Point Link

Network: 203.0.113.0/24

• Total addresses: 256

• Used:

  • Network: 203.0.113.0
  • Broadcast: 203.0.113.255
  • R1: 203.0.113.1
  • R2: 203.0.113.2

• Unused: 252 addresses

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Example 2: Company Needs 5000 Hosts

• Class C → too small (254 hosts)
• Class B → required (65,534 hosts)
• Result: ~60,000 unused addresses

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CIDR (Classless Inter-Domain Routing)

• Introduced by the IETF (Internet Engineering Task Force) in 1993

• Replaced classful addressing

• Removed fixed boundaries:

  • Class A = /8
  • Class B = /16
  • Class C = /24

Why CIDR Matters

CIDR lets you carve networks like a careful sculptor instead of swinging a sledgehammer. Large networks can be split into smaller, efficient subnets.

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Subnetting Example

Base network: 203.0.113.0/24

• Subnet mask: 255.255.255.0
• Usable hosts: 254

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CIDR Subnet Breakdown

/25

• Mask: 255.255.255.128
• Hosts: 2⁷ − 2 = 126

/26

• Mask: 255.255.255.192
• Hosts: 2⁶ − 2 = 62

/27

• Mask: 255.255.255.224
• Hosts: 2⁵ − 2 = 30

/28

• Mask: 255.255.255.240
• Hosts: 2⁴ − 2 = 14

/29

• Mask: 255.255.255.248
• Hosts: 2³ − 2 = 6

/30

• Mask: 255.255.255.252
• Hosts: 2² − 2 = 2

✔ Ideal for point-to-point links (e.g., router-to-router)

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/31

• Hosts: 2¹ − 2 = 0 (traditionally)

However:

• Used for point-to-point links
• No network or broadcast needed

Cisco warning example:

Router(config-if)# ip address 203.0.113.0 255.255.255.254
Warning: use /31 mask on non point-to-point interface cautiously

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/32

• Hosts: 2⁰ − 2 = −1 (conceptually)

Used for:

• Loopbacks
• Static routes
• Identifying a single host

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Key Takeaways

• CIDR enables flexible and efficient IP allocation
• Subnetting reduces waste and improves scalability
• Smaller subnets = better utilization of address space