Overview
The IP address is the logical address that lets data find any host across interconnected networks. IPv4 is the original and still most common version. Understanding IPv4 addresses — their format and structure — is fundamental to everything in networking and network security.
Learning objectives
- Describe the structure of an IPv4 address.
- Explain the network and host portions.
- Understand the scale limitation that led to IPv6.
Core concept
An IPv4 address is a 32-bit number, written as four decimal octets separated by dots, e.g. 192.168.1.10. Each octet ranges from 0 to 255 (8 bits each, 4 × 8 = 32 bits).
An IPv4 address has two conceptual parts:
- Network portion — identifies which network the host is on.
- Host portion — identifies the specific host within that network.
Where the split falls is defined by the subnet mask (covered in the subnetting module). Routers use the network portion to move packets between networks; the host portion pinpoints the device once the right network is reached.
How it works
When a packet is sent, routers examine the destination's network portion to decide the next hop toward that network. Only when the packet reaches the destination network does the host portion identify the exact device. This two-part design is what makes routing scalable — routers can make decisions based on networks rather than tracking every individual host on the internet.
Practical example
Consider 192.168.1.10. On a typical home network, 192.168.1 is the network portion and .10 is the host. Another device might be 192.168.1.11 — same network, different host. A device on a different network, like 10.0.0.5, has a different network portion entirely, so reaching it requires routing between the two networks.
Cybersecurity relevance
IP addresses are central to security: firewalls filter by IP, logs record source IPs, threat intelligence tracks malicious IPs, and access controls allow or deny ranges. Reconnaissance often starts by mapping the IP ranges an organization uses. Reading and reasoning about IPv4 addresses is a daily skill in both offense and defense.
Common mistakes
- Thinking each octet can exceed 255 — 8 bits caps each at 255.
- Ignoring the network/host split — it is what makes routing and subnetting work.
Key takeaways
- An IPv4 address is a 32-bit value written as four octets (0–255), e.g.
192.168.1.10. - It has a network portion (which network) and a host portion (which device), split by the subnet mask.
- Routers use the network portion to route; IP addressing underlies filtering, logging, and recon in security.
Check your understanding
- How many bits is an IPv4 address, and why is each octet limited to 0–255?
- What are the two conceptual parts of an IPv4 address and what does each identify?