IPv4 & IPv6 Addressing and Subnetting

Slide 1 of 14 | CTS1305C | Week 2

IPv4 & IPv6
Addressing & Subnetting

The math behind every network — classes, CIDR, VLSM, and the IPv6 future.

Your company has 200 employees across 4 departments. You have been assigned the 10.10.0.0/16 network. How do you divide it so each department gets its own subnet with room to grow? This presentation teaches the subnetting skills to answer that question.

14 Slides Week 2 IPv4 + IPv6 Subnetting + VLSM

Slide 2 of 14

IPv4 Address Classes

The original classful addressing system (mostly historical but still tested).

Class	First Octet Range	Default Mask	CIDR	Networks	Hosts per Net
A	1-126	255.0.0.0	/8	126	16,777,214
B	128-191	255.255.0.0	/16	16,384	65,534
C	192-223	255.255.255.0	/24	2,097,152	254
D	224-239	N/A	N/A	Multicast (not assigned to hosts)
E	240-255	N/A	N/A	Experimental/reserved

Important

127.0.0.0/8 is reserved for loopback (127.0.0.1 = localhost). Class D is for multicast groups. Class E is reserved. Modern networks use classless (CIDR) addressing, but class concepts appear on every exam.

Slide 3 of 14

Private IP Ranges (RFC 1918)

Addresses that are free to use internally but not routable on the public internet.

Class A Private

10.0.0.0 - 10.255.255.255
/8 — 16.7M addresses. Used in large enterprises. One massive block.

Class B Private

172.16.0.0 - 172.31.255.255
/12 — 1M+ addresses. 16 contiguous /16 blocks. Common in mid-size orgs.

Class C Private

192.168.0.0 - 192.168.255.255
/16 — 65K addresses. The default for home routers and SOHO networks.

NAT

Network Address Translation converts private addresses to public addresses at the network boundary. This is how millions of devices share a small pool of public IPs. Without NAT, IPv4 addresses would have been exhausted decades ago.

Slide 4 of 14

Subnet Masks Explained

The mask separates the network portion from the host portion of an IP address.

AND Operation

IP AND Mask = Network Address. For 192.168.1.100 /24: 192.168.1.100 AND 255.255.255.0 = 192.168.1.0 (the network address).

Host Range

Network address + 1 = first usable host. Broadcast (all host bits = 1) - 1 = last usable host. 192.168.1.1 to 192.168.1.254 (254 hosts).

Slide 5 of 14

CIDR — Classless Inter-Domain Routing

Modern addressing that replaced wasteful classful allocation.

Classful Problem

A company needs 500 hosts. Class C only gives 254. Class B gives 65,534 — wasting 65,000+ addresses. No middle ground.

CIDR Solution

Use any prefix length: /23 gives 510 hosts. Exact fit. No class boundaries. Written as 10.0.0.0/23 — the slash notation replaces classful masks.

CIDR	Subnet Mask	Hosts	Use Case
/24	255.255.255.0	254	Standard LAN subnet
/25	255.255.255.128	126	Small department
/26	255.255.255.192	62	Server VLAN
/27	255.255.255.224	30	Small workgroup
/28	255.255.255.240	14	DMZ, management VLAN
/30	255.255.255.252	2	Point-to-point router links

Formula

Usable hosts = 2^(32 - prefix) - 2. The -2 accounts for the network address (all host bits 0) and broadcast address (all host bits 1).

Slide 6 of 14

Subnetting Example: /24 to /26

Dividing 192.168.1.0/24 into four /26 subnets.

The Math

/24 to /26 = borrowing 2 bits from the host portion. 2^2 = 4 subnets, each with 2^6 - 2 = 62 usable hosts. The block size (increment) is 256 - 192 = 64.

Slide 7 of 14

Subnetting: /24 to /28

16 subnets, each with 14 usable hosts.

The Math

/24 to /28 = borrowing 4 bits. 2^4 = 16 subnets. Each subnet has 2^4 - 2 = 14 usable hosts. Block size: 256 - 240 = 16.

Subnet Boundaries

.0, .16, .32, .48, .64, .80, .96, .112, .128, .144, .160, .176, .192, .208, .224, .240 — each block starts at a multiple of 16.

Subnet	Network	First Host	Last Host	Broadcast
1	192.168.1.0	.1	.14	.15
2	192.168.1.16	.17	.30	.31
3	192.168.1.32	.33	.46	.47
4	192.168.1.48	.49	.62	.63

Use Case

/28 subnets are perfect for DMZs, management VLANs, and small server farms where you need isolation but only have a handful of devices. Waste-free, security-friendly.

Slide 8 of 14

VLSM — Variable-Length Subnet Masks

Different subnet sizes within the same network — maximum efficiency.

VLSM Strategy

Allocate largest subnets first, then carve smaller ones from the remaining space. This prevents overlapping and fragmentation. Always start from the top of the address space and work down.

Slide 9 of 14

IPv6 — 128-Bit Addressing

340 undecillion addresses. Enough for every grain of sand on Earth.

128 Bits

Written as 8 groups of 4 hex digits, separated by colons. 3.4 x 10^38 total addresses — effectively unlimited.

Shortening Rules

Drop leading zeros in each group: 0db8 becomes db8. Replace one contiguous group of all-zero hextets with :: (only once per address).

No Broadcast

IPv6 has no broadcast. Uses multicast (ff02::1 = all nodes) and anycast instead. This reduces unnecessary traffic on the network.

Slide 10 of 14

IPv6 Address Types

Link-local, global unicast, multicast, and loopback.

Global Unicast (GUA)

Prefix: 2000::/3. Routable on the public internet. Equivalent to a public IPv4 address. Assigned by ISPs or RIRs. Typically a /48 to an org, /64 per subnet.

Link-Local (LLA)

Prefix: fe80::/10. Auto-assigned on every IPv6 interface. Not routable beyond the local link. Used for neighbor discovery, routing protocol adjacencies, and default gateway communication.

Multicast

Prefix: ff00::/8. One-to-many delivery. Key addresses: ff02::1 (all nodes), ff02::2 (all routers), ff02::1:ff (solicited-node, used by NDP).

Loopback & Unspecified

::1 = loopback (like 127.0.0.1). :: = unspecified (used during bootstrap when no address is assigned yet).

Slide 11 of 14

SLAAC — Stateless Address Autoconfiguration

IPv6 hosts can configure themselves without a DHCP server.

How It Works

The host receives the /64 prefix from the router's RA and generates its own 64-bit Interface ID (using EUI-64 or a random value for privacy). Then it runs DAD (Duplicate Address Detection) to make sure the address is unique. No DHCP server needed.

Slide 12 of 14

Dual-Stack & Tunneling

Transition strategies for moving from IPv4 to IPv6.

Dual-Stack

Run both IPv4 and IPv6 simultaneously on every device and interface. The preferred approach. Applications choose which protocol to use. Requires double the routing configuration.

Tunneling

Encapsulate IPv6 packets inside IPv4 headers to cross IPv4-only networks. Types: 6to4, ISATAP, Teredo, GRE tunnels. Used when dual-stack is not possible across the whole path.

NAT64

Translates between IPv6 and IPv4 at the network boundary. An IPv6-only host can reach an IPv4-only server through a NAT64 gateway. Used in mobile carrier networks.

Current State

As of 2026, roughly 45% of internet traffic is IPv6. Major cloud providers, mobile carriers, and content networks run dual-stack. Enterprise internal networks are slower to migrate — many still run IPv4-only LANs with dual-stack at the edge.

Slide 13 of 14

IPv4 vs IPv6 Comparison

Side-by-side differences at a glance.

IPv4

32-bit address (4.3 billion)
Dotted-decimal: 192.168.1.1
ARP for address resolution
Broadcast + unicast + multicast
Variable header length (20-60 bytes)
DHCP for auto-configuration
NAT required for conservation
Fragmentation by routers

IPv6

128-bit address (3.4 x 10^38)
Hex-colon: 2001:db8::1
NDP for address resolution
Unicast + multicast + anycast (no broadcast)
Fixed header (40 bytes, extension headers)
SLAAC or DHCPv6
NAT not needed (enough addresses)
Fragmentation only by source

Key Difference

IPv6 was designed with security (IPsec mandatory), mobility, auto-configuration, and simplified headers in mind. IPv4 had these features bolted on after the fact. Both will coexist for years.

Slide 14 of 14

Summary & Key Takeaways

Everything you need to remember from this presentation.

1 IPv4 classful addressing (A/B/C) is historical. Modern networks use CIDR for flexible, classless subnetting.

2 RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. Require NAT for internet access.

3 Usable hosts = 2^(host bits) - 2. Network address (all 0s) and broadcast (all 1s) are reserved.

4 VLSM allows different-sized subnets within the same network. Allocate largest first to avoid overlap.

5 IPv6 uses 128-bit addresses in hex-colon notation. Key types: GUA (2000::/3), Link-Local (fe80::/10), Multicast (ff00::/8).

6 SLAAC lets IPv6 hosts auto-configure using Router Advertisements + self-generated Interface IDs.

7 Transition: dual-stack (preferred), tunneling (6to4, Teredo), NAT64 (translation at boundary).

For our 200-employee company with 10.10.0.0/16: use VLSM. Engineering gets /23, Sales gets /25, HR gets /26, WAN links get /30. Enable dual-stack with a GUA prefix from the ISP. Document everything in the IP address management (IPAM) system.

IPv4 & IPv6 Addressing and Subnetting — Cable Run CTS1305C