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Plain-English explanation

An IPv4 address is a 32-bit number written as four decimal octets (0–255 each), like 192.168.10.45. Every interface on the Internet that must be reached globally needs a unique public address. Inside organizations, private ranges can be reused on isolated networks and translated at the edge with NAT.

RFC 1918 private ranges (know cold):

• 10.0.0.0/8 — one big class A block (10.0.0.0 – 10.255.255.255)
• 172.16.0.0/12 — 172.16.0.0 – 172.31.255.255
• 192.168.0.0/16 — 192.168.0.0 – 192.168.255.255

Deep dive

Special-use ranges (high-yield for CCNA):

| Range | Purpose | |-------|---------| | 127.0.0.0/8 | Loopback (127.0.0.1) | | 169.254.0.0/16 | Link-local / APIPA (no DHCP) | | 224.0.0.0 – 239.255.255.255 | Multicast | | 240.0.0.0+ | Reserved / limited exam focus |

Public vs private: If an address is not in RFC 1918 (and not special-use), it is routable on the public Internet — subject to ISP allocation.

NAT role: Many private hosts share one or a pool of public IPs. The router tracks translations (inside local ↔ inside global). CCNA depth on NAT comes in Week 6 — here, know why private space exists.

Step-by-step — classify addresses

Classify each:

1. 10.50.1.1 → private (10/8)
2. 172.30.5.1 → private (172.16–31)
3. 192.168.99.1 → private (192.168/16)
4. 8.8.8.8 → public
5. 169.254.10.5 → APIPA link-local
6. 127.0.0.1 → loopback

Invalid host examples: network address (host bits all 0), broadcast (host bits all 1) — detailed subnet math in Week 2.

Commands to know

show ip interface brief
show running-config interface GigabitEthernet0/0

Windows
ipconfig /all
Linux
ip addr show

Troubleshooting

| Symptom | Likely issue | |---------|--------------| | 169.254.x.x address | DHCP failure — check server, relay, VLAN | | Duplicate IP warnings | Two hosts same address — ARP flapping | | Cannot reach Internet, local OK | Missing NAT, wrong default route, public routing | | VPN overlap | Both sides use same private range — renumber or NAT |

Plan addressing before mergers and cloud VPC peering — overlap breaks routing even with NAT.

Exam relevance

Practice checklist

• Recite all three RFC 1918 ranges with CIDR notation
• Classify ten random addresses as public, private, or special-use
• Explain why two companies can both use 192.168.1.0/24 internally
• Identify APIPA and loopback addresses in ipconfig output
• Sketch a small LAN with private hosts and one public WAN address