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IPv4 & IPv6 Fundamentals

An IP address is a unique identifier assigned to each device connected to a network. A good analogy is a postal address: it helps data reach the correct destination.

IPv4 (Internet Protocol version 4)

IPv4 uses a 32-bit address. It is written in dotted-decimal format, made of four numbers separated by dots:

192.168.10.25

Each of the four parts is called an octet because it contains 8 bits. With 8 bits, you can represent 256 values (0–255), so 255 is the largest value possible in one octet.

With 32 bits total, IPv4 provides about 2^32 ≈ 4.3 billion unique addresses.

Network and host portions (subnet masks)

An IPv4 address can be thought of as two parts: a network part and a host part. The subnet mask defines where that split happens (for example 255.255.255.0).

This split helps devices and routers determine whether a destination is on the local network or must be reached through routing.

Classful addressing (historical background)

In the early days of IPv4, addresses were grouped into fixed “classes” to simplify allocation. This is called classful addressing. It is important historically, but modern networks use classless addressing.

  • Class A: 1.0.0.0 – 126.255.255.255 (default mask 255.0.0.0)
  • Class B: 128.0.0.0 – 191.255.255.255 (default mask 255.255.0.0)
  • Class C: 192.0.0.0 – 223.255.255.255 (default mask 255.255.255.0)
  • Class D: multicast (224.0.0.0 – 239.255.255.255)
  • Class E: experimental/reserved (240.0.0.0 – 255.255.255.255)

With default masks, the maximum hosts per network were typically:

  • Class A: 16,777,214
  • Class B: 65,534
  • Class C: 254

Private vs public IPv4

As the internet grew, the IPv4 pool started to run out. To extend IPv4, special ranges were reserved as private IP addresses. Private addresses can be reused across different networks because they only need to be unique inside a local network.

The trade-off is that private IP addresses are not routable on the public internet. For internet access, private addresses typically use NAT (explained below).

Private ranges:

  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255

A public IP address is globally unique and routable on the internet, usually assigned by your ISP.

NAT (Network Address Translation)

NAT allows many private devices to share one public IP address. A router translates internal private addresses to a public address when traffic goes out to the internet, and then maps responses back to the correct internal device.

NAT helps conserve public IPv4 addresses and reduces direct exposure of internal addressing, but it does not replace proper firewall rules and security controls.

Subnetting

Subnetting is the process of dividing a larger network into smaller networks (subnets). It helps allocate IP addresses efficiently, reduces unnecessary broadcast traffic, improves performance, and supports segmentation for better security and management.

Classless addressing: VLSM and CIDR

Classful addressing wasted many IP addresses because it forced fixed network sizes. In modern networks, classless addressing is used, where the subnet mask (prefix length) can be chosen to fit the required number of hosts and subnets.

  • VLSM (Variable Length Subnet Masking): use different subnet sizes within the same overall network.
  • CIDR (Classless Inter-Domain Routing): uses prefix notation like 192.168.10.0/24 for allocation and routing.

IPv6 (Internet Protocol version 6)

IPv6 was designed to solve IPv4 exhaustion and improve scalability. IPv6 uses a 128-bit address space, providing an extremely large number of unique addresses (2^128).

IPv6 addresses are written in hexadecimal and separated by colons. Example:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

IPv6 is written as eight groups (often called hextets). Each hextet represents 16 bits and ranges from 0000 to FFFF (where FFFF equals 65,535 in decimal).

IPv4 vs IPv6 (summary)

  • Address size: IPv4 is 32-bit; IPv6 is 128-bit.
  • Format: IPv4 is dotted-decimal; IPv6 is colon-separated hexadecimal.
  • Address space: IPv6 provides vastly more addresses than IPv4.
  • NAT: common in IPv4; less necessary in IPv6 (though still used in some cases).
  • Operations: IPv6 supports cleaner large-scale allocation and routing.

Wrap-up

IPv4 is still widely used, but limited. Private addressing and NAT helped IPv4 scale, while IPv6 provides the long-term solution with a huge address space. Many networks run both during the transition (dual-stack).