IPv4 addressing gives every device a unique 32-bit address so packets can be identified and delivered correctly.
Facts / key topics
- Standard: RFC 791 · Length: 32 bits · Notation: dotted decimal (e.g. 192.168.1.1) · OSI: Layer 3
- Address types: unicast, broadcast, multicast
- CCNA: understand + subnetting + calculations · CCNP: design, summarization, edge cases
- Where: LAN, WAN, campus, DC
Why does it exist?
- Devices must be uniquely identifiable, and routers must know where to send packets.
- Without IPv4 addressing: no routing, no logical communication, only local L2 forwarding (MAC).
- Typical scenarios: LAN segmentation, WAN routing, internet communication.
How does it solve it?
- Every device gets a unique IP address, split into a network portion and a host portion.
- Routers forward on the network portion; the subnet mask defines the boundary.
Step by step
- Init: a host gets an IP (static or DHCP).
- Interpretation: the subnet mask decides – destination local? Or remote → send to default gateway.
- Normal operation: routers forward based on the destination network.
- Edge cases (CCNP): overlapping subnets, wrong summarization, asymmetric routing.
Address classes (historical)
| Class | First octet | Default mask |
|---|---|---|
| A | 1–126 | /8 |
| B | 128–191 | /16 |
| C | 192–223 | /24 |
⚠️ Exam tip: classes are historical only – CIDR replaced them.
CIDR (Classless Inter-Domain Routing)
- Introduces variable prefix lengths: /24, /30, /27
- Enables efficient address use and route summarization
Subnet masks
- Specify which bits are network vs host: 255.255.255.0 = /24; 255.255.255.252 = /30
Subnetting – purpose
- Smaller broadcast domains, better security, more efficient routing.
Private vs public IP
| Type | Range |
|---|---|
| Private | 10.0.0.0/8 |
| 172.16.0.0 – 172.31.255.255 | |
| 192.168.0.0/16 | |
| Public | Globally routable |
Exam tips ⚠️
- “/30” → think point-to-point
- APIPA → DHCP problem
- Overlapping subnets → routing fault
- CIDR always beats classes
Quick summary
- IPv4 = 32 bits, Layer 3 · CIDR replaces classes · subnet mask = network/host split · private IPs routable via NAT · special addresses = exam favourites
| Level | Expectation |
|---|---|
| CCNA | Subnetting + understanding |
| CCNP | Design, summarization, edge cases |