1.4. Addressing: The Three Identities of a Device

💡 First Principle: Every networked device has three types of addresses that serve different purposes: MAC addresses identify the hardware (like a serial number), IP addresses identify the network location (like a mailing address), and port numbers identify the application (like an apartment number). The exam heavily tests when each type is used and how they work together.

What happens when address types are confused: You're troubleshooting connectivity. The user can ping their gateway but can't reach a website. Is it a MAC problem? No—MAC addresses only matter locally. Is it a port problem? Could be—maybe port 443 is blocked. Is it IP/routing? Could be—maybe DNS returns the wrong IP. Understanding which address type operates where tells you what to check and what to ignore.

Consider this scenario: A packet leaves your PC destined for a server across the internet. The IP address stays the same for the entire journey (destination: the server). But the MAC address changes at every router hop—first it's your gateway's MAC, then the next router's, then the next. Port numbers identify the application on both ends. Each address type has a different scope and purpose.

The Three Address Types:

Key Port Numbers for CCNA:

⚠️ Exam Trap: MAC addresses are only used within a single broadcast domain. When a frame crosses a router, the source and destination MAC addresses change, but the IP addresses stay the same (unless NAT is involved).