🌐 01-04: IPv4 Addressing Basics¶

📌 What Is an IP Address?¶

An IP Address (Internet Protocol Address) uniquely identifies a device on a network.

Works like a home address for devices
Used for communication between devices
Example: 192.168.1.1

Without IP addresses, packets would have no way to know where to go or where they came from — every device that wants to send or receive traffic needs one.

🔹 IPv4 vs. IPv6 (Quick Comparison)¶

IPv4 — 32-bit addresses, written as 4 decimal octets (192.168.1.1). Still the most common on most networks today.
IPv6 — 128-bit addresses, written as 8 hexadecimal blocks (2001:0db8:85a3:0000:0000:8a2e:0370:7334). Created because the world ran out of IPv4 addresses.

This lesson focuses on IPv4; see 01-06: IPv6 Basics & Security for the IPv6 deep dive.

🔹 IPv4 Address Structure¶

An IPv4 address consists of:

32 bits total
Divided into 4 octets (8 bits each)
Written in dotted-decimal format

Example: 192.168.60.5

Binary representation: 11000000.10101000.00111100.00000101

Each octet ranges from 0 to 255 (since 8 bits gives 2⁸ = 256 possible values).

🔹 Network ID vs. Host ID¶

Every IP address has two logical parts:

Part	Description
Network ID	Identifies which network the device is on
Host ID	Identifies which device on that network

Example: 192.168.60.5/24

/24 → the first 24 bits are the network portion
The remaining 8 bits are the host portion

This is exactly why two devices need to be on the "same network" (same network ID) to talk directly to each other without a router in between.

🔹 Subnet Mask (The Concept)¶

A subnet mask is how a computer knows where the network part ends and the host part begins. It's a 32-bit pattern of 1s and 0s laid over the IP address:

1 bits → network portion
0 bits → host portion

Example: /22 → 11111111.11111111.11111100.00000000 → 255.255.252.0

💡 Trick worth memorizing: - Network ID → set all host bits to 0 - Broadcast ID → set all host bits to 1

The full math for calculating ranges from a subnet mask is covered in 01-05: IPv4 Subnetting & CIDR.

🔒 Special IP Addresses¶

Private IP Ranges¶

Reserved for use inside private networks — never routed on the public internet.

Range	CIDR
10.0.0.0 – 10.255.255.255	10.0.0.0/8
172.16.0.0 – 172.31.255.255	172.16.0.0/12
192.168.0.0 – 192.168.255.255	192.168.0.0/16

Public vs. Private IP¶

Type	Description
Public IP	Globally unique, routable on the internet
Private IP	Used inside local networks only, not routable

💡 Private IPs need NAT (Network Address Translation) on a router/firewall to reach the internet — this is also a security benefit, since internal hosts aren't directly reachable from the outside.

Loopback Address¶

The loopback address refers to the device itself (localhost):

Range: 127.0.0.0/8
Most common: 127.0.0.1
Packets sent here never leave the device — useful for testing local services.

⚙️ Static vs. Dynamic IP¶

Type	Description
Static IP	Manually assigned, does not change
Dynamic IP	Assigned automatically by DHCP, may change over time

💡 Most home and enterprise client devices use dynamic IPs (via DHCP); servers and network infrastructure usually get static IPs so they can always be found at the same address.

🔍 Viewing and Assigning an IP (Linux)¶

View current addresses:

ip address
# or, a shorter table view:
ip -br addr

Manually assign an IP:

sudo ip addr add 192.168.60.6/24 dev enp0s3

Automatic assignment (DHCP = Dynamic Host Configuration Protocol) hands out an IP address, subnet mask, and default gateway without any manual steps — a DHCP client on the host requests this configuration from a DHCP server on the network.

⚠️ Why Addressing Matters for Security¶

Nearly every network attack in this course depends on IP addressing in some way:

Spoofing (Module 03/04) forges the source IP address in a packet.
Firewalls (Module 05) filter traffic based on source/destination IP.
DNS (Module 06) exists specifically to map names to IP addresses.
VPNs (Module 08) create a private, tunneled IP space over the public internet.

Understanding addressing well is the foundation everything else in this course builds on.

📌 Key Takeaways¶

An IP address uniquely identifies a device for communication.
IPv4 = 32-bit address, 4 octets, dotted-decimal notation.
Every address has a Network ID portion and a Host ID portion, split by the subnet mask/prefix length.
Network ID = all host bits 0; Broadcast ID = all host bits 1.
Private IP ranges (10/8, 172.16/12, 192.168/16) require NAT to reach the internet.
127.0.0.1 is the loopback address — traffic never leaves the device.
DHCP automates IP configuration; static IPs are set by hand.