174 lines
3.3 KiB
Markdown
174 lines
3.3 KiB
Markdown
---
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id: 13. Subnetting (Part 1)
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aliases: []
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tags: []
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---
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# Subnetting (Part 1)
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## IPv4 Address Classes
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| Class | First Octet (Binary) | Range | Default Prefix |
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| ----- | -------------------- | ------- | --------------- |
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| A | 0xxxxxxx | 0–127 | /8 |
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| B | 10xxxxxx | 128–191 | /16 |
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| C | 110xxxxx | 192–223 | /24 |
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| D | 1110xxxx | 224–239 | N/A (Multicast) |
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| E | 1111xxxx | 240–255 | N/A (Reserved) |
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---
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## Maximum Hosts per Network
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* Host bits all **0s** → Network address
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* Host bits all **1s** → Broadcast address
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### Example Calculations
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**192.168.1.0/24 → 192.168.1.255**
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* Host bits: 8
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* Total addresses: 2⁸ = 256
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* Usable hosts: 256 − 2 = **254**
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**172.16.0.0/16 → 172.16.255.255**
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* Host bits: 16
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* Total addresses: 2¹⁶ = 65,536
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* Usable hosts: 65,536 − 2 = **65,534**
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**10.0.0.0/8 → 10.255.255.255**
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* Host bits: 24
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* Total addresses: 2²⁴ = 16,777,216
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* Usable hosts: 16,777,216 − 2 = **16,777,214**
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---
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## How IP Addresses Were Distributed
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* The **IANA (Internet Assigned Numbers Authority)** originally allocated IP ranges based on class.
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* Large organizations received Class A or B networks, while smaller ones received Class C.
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* This rigid system caused significant address waste.
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### Example 1: Point-to-Point Link
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Network: 203.0.113.0/24
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* Total addresses: 256
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* Used:
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* Network: 203.0.113.0
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* Broadcast: 203.0.113.255
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* R1: 203.0.113.1
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* R2: 203.0.113.2
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* **Unused: 252 addresses**
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---
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### Example 2: Company Needs 5000 Hosts
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* Class C → too small (254 hosts)
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* Class B → required (65,534 hosts)
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* Result: ~60,000 unused addresses
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---
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## CIDR (Classless Inter-Domain Routing)
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* Introduced by the **IETF (Internet Engineering Task Force)** in 1993
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* Replaced classful addressing
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* Removed fixed boundaries:
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* Class A = /8
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* Class B = /16
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* Class C = /24
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### Why CIDR Matters
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CIDR lets you carve networks like a careful sculptor instead of swinging a sledgehammer.
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Large networks can be split into smaller, efficient subnets.
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---
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## Subnetting Example
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Base network: **203.0.113.0/24**
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* Subnet mask: 255.255.255.0
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* Usable hosts: **254**
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---
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## CIDR Subnet Breakdown
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### /25
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* Mask: 255.255.255.128
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* Hosts: 2⁷ − 2 = **126**
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### /26
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* Mask: 255.255.255.192
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* Hosts: 2⁶ − 2 = **62**
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### /27
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* Mask: 255.255.255.224
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* Hosts: 2⁵ − 2 = **30**
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### /28
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* Mask: 255.255.255.240
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* Hosts: 2⁴ − 2 = **14**
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### /29
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* Mask: 255.255.255.248
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* Hosts: 2³ − 2 = **6**
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### /30
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* Mask: 255.255.255.252
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* Hosts: 2² − 2 = **2**
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✔ Ideal for point-to-point links (e.g., router-to-router)
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---
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### /31
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* Hosts: 2¹ − 2 = 0 (traditionally)
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However:
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* Used for point-to-point links
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* No network or broadcast needed
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Cisco warning example:
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```cisco
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Router(config-if)# ip address 203.0.113.0 255.255.255.254
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Warning: use /31 mask on non point-to-point interface cautiously
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```
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---
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### /32
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* Hosts: 2⁰ − 2 = −1 (conceptually)
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Used for:
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* Loopbacks
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* Static routes
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* Identifying a single host
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---
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## Key Takeaways
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* CIDR enables flexible and efficient IP allocation
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* Subnetting reduces waste and improves scalability
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* Smaller subnets = better utilization of address space
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