---

id: 1777840095-WYOD
aliases:

* Subnetting (Part 1)
  tags:
* CCNA

# 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)  |

---

## 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**

---

## 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**

A tiny road, but a massive parking lot sitting empty.

---

### Example 2: Company Needs 5000 Hosts

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

---

## 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.

---

## Subnetting Example

Base network: **203.0.113.0/24**

* Subnet mask: 255.255.255.0
* Usable hosts: **254**

---

## 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)

---

### /31

* Hosts: 2¹ − 2 = 0 (traditionally)

However:

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

Cisco warning example:

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

---

### /32

* Hosts: 2⁰ − 2 = −1 (conceptually)

Used for:

* Loopbacks
* Static routes
* Identifying a single host

---

## Key Takeaways

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