IPv4 Address Classes (Simplified)

• The first octet determines the class
• Class A = large networks, Class C = small networks

Network & Host Bits

Every IPv4 address has 32 bits, divided into a network portion and a host portion:

IPv4 Address Classes (Detailed)

Default Subnet Masks

• The subnet mask tells you which portion of the IP address identifies the network and which identifies the host

• 1s in the mask = network portion
• 0s in the mask = host portion

CIDR Notation

• CIDR = Classless Inter-Domain Routing
• Uses "slash" notation to indicate how many bits belong to the subnet mask
• A shortcut for writing the full subnet mask:

• 192.168.1.0/24 = mask 255.255.255.0
• 10.1.0.0/16 = mask 255.255.0.0
• 196.10.10.0/25 = mask 255.255.255.128

Understanding the Power of 2

• We use the power of 2 in IP addressing and subnetting
• It's important to memorize these values

Hosts Per Network = 2^h − 2

• h = number of host bits available
• We subtract 2 because each network reserves:
  • Network address (all host bits = 0)
  • Broadcast address (all host bits = 1)

Public vs Private IP Addresses

Private IP Address Ranges

• Real-World Examples:
  • Your home router typically uses 192.168.1.0/24 or 192.168.0.0/24 (Private Class C)
  • Large corporate networks often use 10.0.0.0/8 (Private Class A) for millions of internal devices
  • Cloud providers like AWS use 172.16.0.0/12 ranges for Virtual Private Clouds

Public vs Private — Visualized

The Loopback Address

• 127.0.0.0 – 127.255.255.255 is reserved for loopback (a host's own address)
• 127.0.0.1 is the default loopback address on most operating systems
• Used for diagnostics — checks that TCP/IP is correctly installed:
  • The OS loops the packet back to itself without touching the NIC
  • Data is forwarded to a virtual network interface inside the OS
• If you can ping it, TCP/IP is working:
  • ping 127.0.0.1
  • ping localhost
  • ping loopback

Why Subnet?

• Using default Class A, B, and C subnets (Classful IP Addressing) is inefficient:
  • Wastes unused IP addresses (especially public ones)
• Subnetting lets you create multiple logical networks within a single Class A, B, or C network:
  • Breaks up larger networks into multiple smaller sub-networks (subnets)
• Allows more efficient routing via router summarization
• Increased network security — isolate departments, devices, or traffic

Fixed-Length Subnetting (FLSM)

• We will learn fixed-length subnet mask (FLSM) — all subnets are the same size
• There is also variable-length subnetting (VLSM), which is beyond the scope of this course

Class C Subnetting Example

You're the network admin for a university's Computer Science department:

• 4 new lecture halls, each needing a 60-person wireless network
• Assigned: 200.15.178.0 — a Class C network (254 hosts by default)
• How do you split this one network into 4 smaller subnets?

Process of Subnetting

• We borrow host bits to create more sub-networks from a Class A, B, or C network
• When you borrow host bits:
  • You create additional subnets
  • You decrease the number of host addresses available per subnet

Class C example (24 network bits + 8 host bits):

After borrowing 2 bits:

How to Create Subnets

• Borrow bits from the host portion of an IP address
• Each bit borrowed doubles the number of subnets:
  • Borrow 1 bit → 2¹ = 2 subnets
  • Borrow 2 bits → 2² = 4 subnets
  • Borrow 3 bits → 2³ = 8 subnets
  • Borrow 4 bits → 2⁴ = 16 subnets
• Number of subnets = 2^x where x = bits borrowed

Creating Subnets — Visualized

Subnetting Questions

To create a subnet, answer these two questions:

Class C — Possible Subnets

• x = number of host bits borrowed to create subnets
• y = remaining host bits (y = 8 − x for Class C)
• Block size = 2^y = the increment between subnet network addresses

Class B — Possible Subnets

Class A — Possible Subnets

Subnet Count: 2^x

Where x = number of host bits borrowed:

Hosts per Subnet: 2^y − 2

Where y = host bits remaining:

Subnetting Example

1. How many host bits do we need to borrow?
2. How many host addresses per subnet?
3. What are the valid subnets?
4. What is the new subnet mask?
5. For each subnet, what is the:
   • Network Address
   • First Host IP
   • Last Host IP
   • Broadcast Address

Subnetting Example — Solution

Subnetting Example 2

1. How many host bits do we need to borrow?
2. How many addresses/hosts per subnet?
3. What are the valid subnets?
4. What is the new subnet mask?
5. For each subnet, what is the:
   • Network Address
   • Host IP Range
   • Broadcast Address

Subnetting Example 2 — Solution

Week 5 — Summary