2.1Private and public addresses

On the support floor, being able to glance at the IP address a PC shows and read "is this an internal-only address or one that can reach the internet?" and "was it even handed out properly by DHCP?" makes isolation far faster. This section covers the difference between a globally unique public address and a private address reused within an organization (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), the idea of address classes A/B/C, the APIPA address (starting with 169.254) that appears when DHCP does not answer, and NAT that bridges private and public—learned not as rote facts but as the ability to read "what this address means."

2.1.1Public and private addresses

• A public address is a globally unique address on the internet that is routed as-is. It is managed and allocated through an ISP or a regional registry, so you cannot just assign one yourself.
• A private address may be freely reused inside an organization (reserved by RFC1918). The same block such as 192.168.0.0/16 is used in homes and offices worldwide, yet they do not collide because a private address is not routed directly on the internet. Reaching the outside requires the NAT described later.
• A first step in telling them apart: if it starts with 10., 172.16-172.31., or 192.168., it is private; 169.254. is the APIPA below (a sign of DHCP failure); other assignable addresses are public.

2.1.2Address classes, private ranges, and APIPA

• IPv4 has a history of dividing into address classes by the first octet: Class A (1-126, default /8), Class B (128-191, default /16), and Class C (192-223, default /24). Beyond these, D (224-239) is multicast and E is experimental. For an introduction, knowing A/B/C and their default masks is enough.
• Each class reserves a private range: 10.0.0.0/8 (Class A, large internal networks), 172.16.0.0/12 (Class B, 172.16 through 172.31), and 192.168.0.0/16 (Class C, homes and small sites). This is why devices behind a home router are usually 192.168.x.x.
• APIPA (169.254.0.0/16) is an automatic assignment the OS gives itself when it could not obtain an address from DHCP. If an IP starts with 169.254, that is a strong hint that "DHCP is not reaching it," so first suspect the cable, the switch, or the DHCP server.

2.1.3The role of NAT (address translation)

• NAT (Network Address Translation) is the mechanism by which a border router rewrites an internal private address into a public address to reach the internet, then reverses the translation on the return traffic to deliver it to the correct internal device. It solves the problem that a private address cannot reach the outside as-is.
• In homes and small offices, many devices commonly share one (or a few) public address to reach the outside, distinguished by port numbers (a form called PAT or IP masquerade). This also conserves scarce IPv4 addresses.

Suppose the help desk gets a call: "only the newly brought-in laptop cannot connect to anything, internal or external." You check the address with ipconfig (Windows) and find the IP is 169.254.24.7. This single number narrows the cause dramatically. An address starting with 169.254 is APIPA—that is, an automatic assignment the PC gave itself because it could not obtain an address from DHCP. So rather than suspecting the PC's IP settings themselves, you should chase "why is no DHCP response arriving?" You check in order: the LAN cable and switch port (physical/L2), then whether that port is in the right VLAN, and whether the DHCP server or relay is running. Conversely, if the IP were handed out correctly in a private range like 192.168.10.34 yet only the outside is unreachable, the cause moves to a different layer. Because a private address cannot reach the internet as-is, the focus becomes whether NAT at the border (and the default gateway) is working. The key point: if you hold the reading "169.254 = DHCP failure," "10./172.16-31./192.168. = a correctly issued private address that needs NAT to go outside," and "other assignable addresses = public," then a single IP display tells you where to check next. An address is not something to memorize but a map that translates a symptom into a place.

2.1.4Section summary

• A public address is globally unique and routable; a private address (10/8, 172.16/12, 192.168/16) is reused internally and cannot reach the outside directly
• APIPA (169.254.0.0/16) is an automatic assignment on DHCP failure and is a strong hint of a connectivity problem
• NAT translates private to public at the border, letting many internal devices share a few public addresses to reach the outside