IPv4 Addressing and Subnetting IPv4 address classes, CIDR, and subnetting
IPv4 Address Classes
| Class | Range | Default Mask | Use |
|---|
| A | 1.0.0.0 – 126.255.255.255 | /8 (255.0.0.0) | 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 | N/A | Multicast |
| E | 240.0.0.0 – 255.255.255.255 | N/A | Reserved/Experimental |
Private Address Ranges (RFC 1918)
- 10.0.0.0/8 — Class A private (10.0.0.0 – 10.255.255.255)
- 172.16.0.0/12 — Class B private (172.16.0.0 – 172.31.255.255)
- 192.168.0.0/16 — Class C private (192.168.0.0 – 192.168.255.255)
Subnetting Key Formula
- Subnets = 2^(borrowed bits) | Hosts = 2^(host bits) – 2
- /30 = 255.255.255.252 → 4 addresses, 2 usable (point-to-point links)
- /29 = 255.255.255.248 → 8 addresses, 6 usable
- Broadcast = last address in subnet; Network = first address
Exam Focus Areas
- 127.0.0.0/8 is loopback (localhost = 127.0.0.1)
- 169.254.x.x is APIPA (link-local) — no DHCP response
- /32 = single host route; /31 = point-to-point (RFC 3021)
- VLSM allows different subnet masks within same network
