# Addressing, Subnetting, and CIDR

IP addressing is how devices identify and reach each other across networks. Subnetting allows you to divide networks efficiently.

## IPv4 Addressing

IPv4 addresses are 32-bit, written in dotted decimal notation (e.g., 192.168.1.10). Each octet is 8 bits (0-255).

### Address Classes (Historical)

| Class | Range | Default Mask | Use Case |
|---|---|---|---|
| **A** | 1.0.0.0 - 126.255.255.255 | /8 (255.0.0.0) | Very large networks |
| **B** | 128.0.0.0 - 191.255.255.255 | /16 (255.255.0.0) | Medium networks |
| **C** | 192.0.0.0 - 223.255.255.255 | /24 (255.255.255.0) | Small networks |
| **D** | 224.0.0.0 - 239.255.255.255 | Multicast | Multicast |
| **E** | 240.0.0.0 - 255.255.255.255 | Reserved | Experimental |

*Classful addressing is mostly obsolete; CIDR is used today.*

### Private Address Ranges (RFC 1918)

| Range | CIDR | Use Case |
|---|---|---|
| **10.0.0.0 - 10.255.255.255** | 10.0.0.0/8 | Large private networks |
| **172.16.0.0 - 172.31.255.255** | 172.16.0.0/12 | Medium private networks |
| **192.168.0.0 - 192.168.255.255** | 192.168.0.0/16 | Home/small networks |

### Special Addresses

| Address | Purpose |
|---|---|
| **127.0.0.0/8** | Loopback (localhost) |
| **169.254.0.0/16** | APIPA/link-local (auto-config failure) |
| **255.255.255.255** | Limited broadcast |
| **0.0.0.0** | Default route / unspecified |

## CIDR (Classless Inter-Domain Routing)

CIDR replaced classful addressing, allowing flexible subnet sizes with prefix notation.

**Notation:** `192.168.1.0/24` means first 24 bits are network, last 8 are host.

### CIDR Cheat Sheet

| CIDR | Subnet Mask | Hosts | Notes |
|---|---|---|---|
| /32 | 255.255.255.255 | 1 | Single host |
| /31 | 255.255.255.254 | 2 | Point-to-point links (RFC 3021) |
| /30 | 255.255.255.252 | 2 usable | Point-to-point |
| /29 | 255.255.255.248 | 6 usable | Small networks |
| /28 | 255.255.255.240 | 14 usable | Small networks |
| /27 | 255.255.255.224 | 30 usable | Small networks |
| /26 | 255.255.255.192 | 62 usable | Medium |
| /25 | 255.255.255.128 | 126 usable | Medium |
| /24 | 255.255.255.0 | 254 usable | Common |
| /23 | 255.255.254.0 | 510 usable | Larger |
| /22 | 255.255.252.0 | 1022 usable | Larger |
| /21 | 255.255.248.0 | 2046 usable | Larger |
| /20 | 255.255.240.0 | 4094 usable | Larger |
| /16 | 255.255.0.0 | 65534 usable | Large |
| /8 | 255.0.0.0 | 16M+ usable | Very large |

*Usable hosts = 2^(host bits) - 2 (network and broadcast addresses)*

## Subnetting Example

**Problem:** Split 192.168.10.0/24 into 4 subnets

**Solution:**
- Need 4 subnets = 2 bits borrowed
- New prefix = /26
- Subnets:
  - 192.168.10.0/26 (0-63)
  - 192.168.10.64/26 (64-127)
  - 192.168.10.128/26 (128-191)
  - 192.168.10.192/26 (192-255)

Each subnet has 62 usable hosts.

## VLSM (Variable Length Subnet Masking)

VLSM allows different subnet sizes within the same network, avoiding waste.

**Example:** Need subnets for different departments with different sizes:
- IT: 50 hosts → needs /26 or /25
- Sales: 20 hosts → needs /27
- Management: 5 hosts → needs /29
- Point-to-point links: 2 hosts each → need /30

VLSM lets you allocate efficiently instead of giving everyone the same size.

## IPv6 Basics

IPv6 addresses are 128-bit, written in hexadecimal with colons (e.g., 2001:0db8:85a3::8a2e:0370:7334). Much larger address space.

| Feature | IPv4 | IPv6 |
|---|---|---|
| **Length** | 32-bit | 128-bit |
| **Notation** | Dotted decimal | Hex colon |
| **Addresses** | ~4.3 billion | ~340 undecillion |
| **NAT** | Common | Usually not needed (end-to-end) |
| **Auto-config** | DHCP or manual | SLAAC (Stateless Address Autoconfiguration) |
| **Header** | Variable, complex | Fixed 40-byte header |

### Key IPv6 Concepts
- **Link-local**: fe80::/10 (auto-configured, local link only)
- **Unique local (ULA)**: fc00::/7 (private equivalent)
- **Global unicast**: 2000::/3 (public)
- **Multicast**: ff00::/8 (replaces broadcast)
- **Loopback**: ::1/128

## Practical Tips

1. **Memorize common prefixes**: /24, /25, /26, /27, /28 are most common
2. **Use binary when learning**: Helps understand why
3. **Practice with calculators**: ipcalc, subnet calculators are fine tools
4. **Think in powers of 2**: 2^n for hosts/subnets
5. **In cloud**: You'll work with VPC CIDRs, subnets, route tables constantly

## Related Notes

- [[Foundations/OSI-TCPIP-Models.md]] — Where IP fits
- [[Routing/Routing-Fundamentals.md]] — How packets are routed
- [[Cloud-Modern-Networking/VPC-Fundamentals.md]] — Cloud addressing
- [[Network-Services/NAT-And-PAT.md]] — Address translation

## Sources

* RFC 1918 (Address Allocation for Private Internets)  
* RFC 4632 (Classless Inter-domain Routing)  
* RFC 4291 (IP Version 6 Addressing Architecture)  
* Kurose & Ross - *Computer Networking*