Update 13. Subnetting (Part 1).md
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---
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id: 1777840095-WYOD
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aliases:
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- Subnetting (Part 1)
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tags:
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- CCNA
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---
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* Subnetting (Part 1)
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tags:
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* CCNA
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# Subnetting (Part 1)
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## IPV4 Address Classes
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## IPv4 Address Classes
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| Class |First octet | numeric range | Prefix Length |
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| ----- | ---------- | ------------- |------------- |
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| A |0xxxxxxxx | 0-127 | /8 |
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| B |10xxxxxxx | 128-191 | /16 |
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| C |110xxxxxx | 192-223 | /24 |
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| D |1110xxxxx | 224-239 | |
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| E |1111xxxxx | 240-255 | |
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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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## Maximum Hosts per Network
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Host portion all 0s = Network address (network ID)
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Host portion all 1s = broadcast address
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* Host bits all **0s** → Network address
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* Host bits all **1s** → Broadcast address
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192.168.1.0/24 -> 192.168.1.255/24
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Host portion = 8 bits = 2^8 = 256
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Maximum hosts per network = 2^8-2 = 254
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### Example Calculations
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**192.168.1.0/24 → 192.168.1.255**
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172.16.0.0/16 -> 172.16.255.255/16
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Host portion = 16 bits = 2^16 = 65,536
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Maximum hosts per network = 2^16 -2 = 254
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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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10.0.0.0/8 -> 10.255.255.255/8
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Host portion = 24 bits = 2^24 = 16,777,216
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Maximum hosts per network = 2^24 -2 = 16,777,214
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**172.16.0.0/16 → 172.16.255.255**
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### How IP addresses are distrubuted
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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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- The [[IANA]] (Internet Assigned Numbers Authority) assigns IPV4 addresses/networks to
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companies based on their size
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- For example, a very large company might receive a class A or class B network, while a small
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company might receive a ckass C network.
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- However, this led to many wasted IP addresses.
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**10.0.0.0/8 → 10.255.255.255**
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#### Example 1
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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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| R1 |<----->| R2 |
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+----+ ^ +----+
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Point to Point network
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- 256 addresses
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- 1 network address (203.0.113.0)
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- 1 broadcast address (203.0.113.255)
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- 1 R1 address (203.0.113.1)
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- 1 R2 address (203.0.113.2)
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- **252 addresses Wasted**
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---
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## How IP Addresses Were Distributed
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#### Example 2
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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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Company X needs IP addressing for 5000 end hosts.
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- A class C network does not provide enough addresses, so a class B network must be assigned
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- This will result in about 6000 addresses being wasted.
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### Example 1: Point-to-Point Link
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## CIDR
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Network: 203.0.113.0/24
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- When the internet was first created, the creators did not predict that the internet
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would become as large as it is today
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- This resulted in wasted address space like the examples.
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- The [[IETF]] introduced [[CIDR]] in 1993 to replace the 'classful' addressing system.
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- With CIDR, the requirement of ...
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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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were removed
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- This allowed larger networks to be split into smaller networks, allowing greater efficiency
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- These smaller networks are called 'subnetworks' or [[subnetting]]
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* Total addresses: 256
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* Used:
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+----+ +----+
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| R1 |<----->| R2 |
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+----+ ^ +----+
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203.0.113.0/24
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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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Network address
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11001011.00000000.01110001.*00000000*
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203 . 0 . 113 . 0
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A tiny road, but a massive parking lot sitting empty.
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Subnet Mask
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11111111.11111111.11111111.*00000000*
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255 . 255 . 255 . 0
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---
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2^8 - 2 = 254 usable addresses.
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### Example 2: Company Needs 5000 Hosts
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How many usable addreses are there in each network?
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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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#### CIDR /25
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**203.0.113.0/25**
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Network address:
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11001011.00000000.01110001.0*0000000*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.1*0000000*
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255 . 255 . 255 . 128
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*2^7 - 2 = 126 usable addresses*
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---
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#### CIDR /26
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**203.0.113.0/26**
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Network address
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11001011.00000000.01110001.00*000000*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.11*000000*
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255 . 255 . 255 . 192
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*2^6 - 2 = 62 usable addresses*
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## CIDR (Classless Inter-Domain Routing)
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#### CIDR /27
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**203.0.113.0/27**
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Network address
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11001011.00000000.01110001.00*000000*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.111*00000*
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255 . 255 . 255 . 224
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*2^5 - 2 = 30 usable addresses*
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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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#### CIDR /28
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**203.0.113.0/28**
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Network address
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11001011.00000000.01110001.00*000000*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.1111*0000*
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255 . 255 . 255 . 240
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*2^4 - 2 = 14 usable addresses*
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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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#### CIDR /29
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*203.0.113.0/29*
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Network address
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11001011.00000000.01110001.00000*000*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.11111*000*
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255 . 255 . 255 . 248
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*2^4 - 2 = 6 usable addresses*
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### Why CIDR Matters
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#### CIDR /30
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**203.0.113.0/30**
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Network address
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11001011.00000000.01110001.000000*00*
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203 . 0 . 113 . 0
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Subnet Mask
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11111111.11111111.11111111.111111*00*
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255 . 255 . 255 . 252
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*2^4 - 2 = 6 usable addresses*
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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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This is the perfect CIDR for our 2 router
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---
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The remaining addresses in the 20 203.0.113.0/24 address block (20 203.0.113.4 - 20 203.0.113.255)
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are now available to be used in other subnets!
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## Subnetting Example
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#### CIDR /31
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*203.0.113.0/31*
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Base network: **203.0.113.0/24**
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2^1 -2= 0 usable addresses
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* Subnet mask: 255.255.255.0
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* Usable hosts: **254**
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but for our point to point connection we can utilisize it
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no need for broadcast or network address
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In cisco device however you get a systeme warning
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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-pint interface cautiously
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Warning: use /31 mask on non point-to-point interface cautiously
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```
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The remaining addresses in the 20 203.0.113.0/24 address block (20 203.0.113.2 - 203.0.113.255)
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are now available to be used in other network!
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---
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### /32
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#### CIDR /32
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*203.0.113.0/32*
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* Hosts: 2⁰ − 2 = −1 (conceptually)
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2^1 -2= -1 usable addresses
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Used for:
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can be used in static routes
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don't worry about them now
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* Loopbacks
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* Static routes
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* Identifying a single host
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## Review
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---
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- [[CIDR]] (Classless Inter-Domain Routing)
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- The process of subnetting
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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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