IP Addressing & Subnetting

Overview

IP addressing and subnetting are how individual devices get identified and organized within and across networks — core knowledge for both Network+ and Security+.

Key concepts

• IPv4 vs IPv6
• Public vs private vs loopback addresses
• Default gateway, subnet mask
• CIDR notation
• Network address vs broadcast address vs usable hosts

How it works

IP Addressing

An IP address is a logical address assigned to a device on a network so information can be sent to the correct destination.

IPv4

Uses 32-bit addresses written as four decimal numbers separated by periods. Has a limited number of available addresses.

IPv6

Uses 128-bit addresses, providing a much larger address space than IPv4. Addresses use hexadecimal values separated by colons.

Public IP Address

Reachable through the internet — ISPs assign these to internet-connected networks and services.

Private IP Address

Used inside a local network — not directly routable across the public internet.

Loopback Address

Refers to the same device that sends the traffic — used to test network software and services locally.

Default Gateway

Sends traffic from a local network to other networks — commonly a router.

Subnet Mask

Separates the network portion of an IPv4 address from the host portion, helping a device determine whether another address is local or on a different network.

Subnetting

Subnetting divides a larger network into smaller networks, improving organization, reducing broadcast traffic, and separating groups of devices for performance and security.

CIDR Notation

Classless Inter-Domain Routing — uses a slash and number to show how many bits are used for the network portion of an address (e.g. /24).

Network Address

Identifies the subnet itself — cannot normally be assigned to an individual device.

Broadcast Address

Sends traffic to every device within the same subnet — cannot normally be assigned to an individual device.

Usable Host Addresses

The addresses actually available for devices on a subnet — the count depends on the subnet mask/CIDR prefix length.