Every IP packet on the planet starts and ends with two 32-bit numbers - the source and destination addresses. CCNA gives you the structure to read those numbers, decide what is local and what is remote, and what your router does about it. This lab builds the structural intuition on top of the running base topology, using R2 (which sits between two segments) as the example device.
One router, two interfaces, one set of show commands that explain the entire address model.
What you will learn
- The five IPv4 classes (A, B, C, D, E) - what they mean and why almost no one uses them in 2026
- RFC 1918 private address space and where it lives in real enterprise networks
- Subnet masks and CIDR notation - the difference between
/24and255.255.255.0is just decoration - Network address vs. host addresses vs. broadcast address inside a subnet
- The ARP table, the special role of loopback interfaces, and why
show ip interfaceis a lot more interesting thanshow ip interface brief
What this lab does NOT cover
- Variable-length subnet masking (VLSM). That is the next lab, nf-04.
- IPv6. That is nf-05.
- NAT and address translation. That is the entire IP Services pillar.
- Routing protocols. Connected routes only here.
The five classes (mostly historical context)
| Class | First octet range | Default mask | Hosts per network | 2026 relevance |
|---|---|---|---|---|
| A | 1 - 126 | /8 (255.0.0.0) | ~16.7 million | Mostly carved into smaller blocks via CIDR. Class A is no longer used in default form anywhere. |
| B | 128 - 191 | /16 (255.255.0.0) | ~65,000 | Similar story. Big enterprises still allocate /16 worth of private space; rarely classful. |
| C | 192 - 223 | /24 (255.255.255.0) | 254 | The size most LANs are built on, but the /24 is a deliberate CIDR allocation, not an artifact of classful routing. |
| D | 224 - 239 | n/a | n/a | Multicast group addresses. Still used for routing protocols (OSPF 224.0.0.5, EIGRP 224.0.0.10) and streaming. |
| E | 240 - 255 | n/a | n/a | Reserved. Hands off. |
Classful routing died in the late 1990s. CIDR (Classless Inter-Domain Routing) replaced it. The classes survive as exam trivia and as the basis for default masks when you type an address without a mask, but nothing in a modern network actually treats 10.0.0.0 as a Class A network. It is treated as whatever the prefix length says it is.
RFC 1918 private address space
The three blocks every enterprise network uses internally. None of these are routed on the public internet - your edge router or firewall translates them via NAT before traffic leaves.
| Block | Range | CIDR | Common use |
|---|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 - 10.255.255.255 | One /8 | Large enterprises. Plenty of room for hierarchical addressing. |
| 172.16.0.0/12 | 172.16.0.0 - 172.31.255.255 | 16 /16 blocks | Mid-size networks, often by accident because it was the default on someone's home router 15 years ago. |
| 192.168.0.0/16 | 192.168.0.0 - 192.168.255.255 | 256 /24 blocks | Small office and home networks. Your home Wi-Fi router almost certainly hands out 192.168.x.x. |
PingLabz labs use 10.0.0.0/8 throughout. The convention is laid out in the PingLabz lab IP scheme.
