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📘 08-11: Subnets in IPv6

Network Systems

Module 08: Subnets and VLANs

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❓ What is IPv6 Subnetting?

IPv6 subnetting is the process of dividing an IPv6 network into smaller sub-networks.

Unlike IPv4, IPv6 provides a vast address space (128 bits), making subnetting simpler and more flexible.

🌐 IPv6 Address Structure

An IPv6 address consists of 128 bits, divided into 8 blocks (hextets), each containing 16 bits (16 × 8 = 128 bits).

An IPv6 address is logically divided into:

  • Network prefix / Site Prefix (global routing prefix + subnet ID)
  • Interface ID (host portion)

Example:

2001:0db8:abcd🔢0000:0000:0000:0001/64
(In short: 2001:db8:abcd🔢:1/64)

  • First 64 bits → Network prefix (2001:db8:abcd:1234)
  • Last 64 bits → Interface ID (0000:0000:0000:0001)

🧮 Standard IPv6 Subnet Size

The most common subnet size in IPv6 is:

👉 /64

This means: - 64 bits → network
- 64 bits → host

💡 Why /64 is Standard

  • Required for many IPv6 features (e.g., SLAAC)
  • Provides a huge number of addresses
  • Simplifies network design

Number of addresses in /64: - 2⁶⁴ (extremely large)

🧮 Subnetting in IPv6

Subnetting in IPv6 is done by extending the network prefix.

Example:

  • Original network: 2001:db8::/32

We can create subnets by increasing the prefix:

  • 2001:db8:0001::/48
  • 2001:db8:0002::/48
  • 2001:db8:0003::/48

👉 Each /48 is a separate subnet

🧮 Example: /48 to /64 Subnetting

Given: - Network: 2001:db8::/48

We want to create /64 subnets.

🧮 Step 1: Calculate Available Subnet Bits

  • Original prefix = /48
  • New prefix = /64

Borrowed bits = 64 − 48 = 16

🧮 Step 2: Number of Subnets

Number of subnets = 2¹⁶ = 65,536 subnets

🧮 Step 3: Subnet Structure

The 16 bits are used in the 4th hextet.

Example subnets:

  • 2001:db8:0000:0000::/64
  • 2001:db8:0000:0001::/64
  • 2001:db8:0000:0002::/64
  • ...
  • 2001:db8:0000:ffff::/64

🧮 Key Differences: IPv4 vs IPv6 Subnetting

Feature IPv4 IPv6
Address size 32-bit 128-bit
Subnetting Complex Simpler
Typical subnet Variable /64
Address shortage Yes No
Need for optimization High Low

🌐 Important Concepts in IPv6

🔹 No Broadcast Address

IPv6 does not use broadcast addresses.

Instead, it uses: - multicast
- anycast

🔹 Huge Address Space

Each subnet (/64) contains: - 2⁶⁴ addresses

👉 No need to worry about address wastage

🔹 Hierarchical Addressing

IPv6 supports structured allocation:

  • Global routing prefix
  • Subnet ID
  • Interface ID

This improves: - routing efficiency
- scalability

📌 Practical Example

Question:
An organization is assigned: 2001:db8::/48

It wants to assign a separate subnet to each department.

  • What is the maximum number of departments it can support?
  • How many hosts are possible per department?

Answer:

  • Subnetting to /64 → borrow 16 bits
  • Maximum number of departments = 2^16 = 65,536

👉 Each department gets one /64 subnet

  • Number of hosts per department = 2^64

👉 Each subnet supports 2^64 host addresses (practically unlimited)

💡 Key Idea

IPv6 subnetting focuses on simplicity and scalability,

👉 using large, fixed-size subnets (/64) instead of complex calculations like IPv4.