Internet Protocol v4 (IPv4)

Monday, March 9, 2026

Lesson Objectives

1. Understand IPv4 datagram structure and header fields.
2. Describe IPv4 addressing: classes, subnetting, and CIDR notation.
3. Explain how DHCP assigns IP addresses dynamically.
4. Calculate subnet masks, number of hosts, network/broadcast addresses.

Preparation

• \(4.3.1\) IPv4 Datagram Format
• \(4.3.2\) IPv4 Addressing

• Understand the structure of an IPv4 datagram, including its header fields and their purposes.
• Learn about IPv4 addressing, including the concept of classes, subnetting, and CIDR notation.
• Understand how Dynamic Host Configuration Protocol (DHCP) works to assign IP addresses dynamically to hosts in a network.
• Calculate subnet masks, determine the number of hosts in a subnet, and identify the network and broadcast addresses for a given IPv4 address and subnet mask.

IPv4 Datagram

Addressing

What is an IP Address?

• A 32-bit number that uniquely identifies each host interface on a network.
• Written in dotted-decimal notation, e.g. \(192.168.100.200\)
• Logical address - it can change!
• Assigned by network administrators or dynamically via DHCP.
• Can you assign it yourself?

Network Interface

• 🔌 Connection point between a host and a physical link.

• 🛣️ What about a router?
• 🖥️ Is a router also a host?
• 🔁 Can a host have multiple interfaces?
• 💻📶 How many actual physical interfaces does your laptop have?

Reading IPv4 Addresses

• Each octet represents 1 byte (8 bits) of the address
• Range: \(0 \rightarrow (2^8 - 1) = 255\)

\[ \def\eightblanks{{\_\space\_\space\_\space\_\space\_\space\_\space\_\space\_}} \def\fourtimeseightblanks{\eightblanks\cdot\eightblanks\cdot\eightblanks\cdot\eightblanks} \]

\[199.239.136.245\]

\[\fourtimeseightblanks\]

\[192.168.100.1\]

\[\fourtimeseightblanks\]

Who is in charge of assigning IP addresses? IANA

Reserved Address Spaces

• Cannot be used on the public Internet
• Used for internal communication within private networks

Private Networks

• Class A: 10.0.0.0/8

• Class B: 172.16.0.0/12

• Class C: 192.168.0.0/16

• Used with NAT to reach the Internet

Special-Purpose Addresses

• Loopback: 127.0.0.0/8
• Link-local: 169.254.0.0/16
• Multicast: 224.0.0.0/4
• Reserved: 240.0.0.0/4

Where to find an IP Address?

• 🔌 I just plugged in my laptop to the network.
• 🤔 How do I get an IP address?
• 🙋 Do I have to ask the network administrator for one?
• 🎲 Can I just assign myself one?
• 🚶 What if I move to a different network?
• 📋 Can someone just keep track of all the IP addresses and tell me which one to use?

Dynamic Host Configuration Protocol (DHCP)

Messages: DORA

• DISCOVER: Who is in charge here of assigning IP addresses?
• OFFER: I am the DHCP server. Would you like x.x.x.x?
• REQUEST: Yes, I will take x.x.x.x
• ACK: Great, I will write that down. You have the lease for 86400 seconds.

Leases

• Time to live (TTL) for each assigned IP address
• Matches physical address (MAC address) to the logical address (IP address)
• Can be released or renewed; you can request the same one again

Subnetting

Broadcast Domain

• 💻 A group of devices that can communicate directly without a router
• 📣 Yelling: all devices receive broadcast messages sent by others
• ⚡ Direct communication is efficient
• ❓ Who broadcasts? ARP, DHCP etc.
• 🚧 Routers separate broadcast domains, so they do not forward broadcast traffic
• 🔊 Very noisy if the broadcast domain is too large
• ⚠️ Also very dangerous if not properly secured

Subnet

• 🌐 Smaller portion of a larger network
• ✂️ Used to break up large IP spaces into smaller ones - reduce wastage
• 👥 Defined by people in charge of the network

\[200.23.20.0/24 \rightarrow 24 \text{ network bits} + 8 \text{ host bits} = \underset{\text{NETWORK ADDRESS}}{\underline{11001000 \cdot 00010111 \cdot 00010100}} \cdot \underset{\text{HOST}}{\underline{00000000}}\]

Network Address

• Reserved part of the address
• Same for all devices in the subnet
• Limits broadcast domain size

Host Portion / Part / Number / Identifier

• Remaining bits after the network prefix
• Unique to each device within the subnet
• Used to identify individual hosts

How many bits?

Classless Inter-Domain Routing (CIDR) Notation

\[\text{Network ID} = \underset{\text{NETWORK ADDRESS}}{\underline{10.22.128.0}} / \underset{\text{CIDR}}{\underline{22}} \rightarrow 22 \text{ network bits} + 10 \text{ host bits}\]

Subnet Masks

• 32-bit number with a bunch of 1s followed by a bunch of 0s; matches CIDR notation
• Tells you how many bits are in the network vs host portions

\[\text{Network ID} = \underset{\text{NETWORK ADDRESS}}{\underline{00001010 \cdot 00010110 \cdot 100000}} \space \underset{\text{HOST}}{\underline{00 \cdot 00000000}}\] \[\text{Subnet Mask} = \underset{\text{22 NETWORK BITS}}{\underline{11111111 \cdot 11111111 \cdot 111111}} \space \underset{\text{10 HOST BITS}}{\underline{00 \cdot 00000000}}\]

\[= 255.255.252.0\]

How many hosts?

\[\text{Total Possible Addresses} = 2^{\text{Number of Host Bits}}\]

First Address: Network ID

• Reserved for the subnet itself
• All host bits are 0

Last Address: Broadcast

• Send messages to all hosts in the subnet
• All host bits are 1

\[\text{Usable Host Addresses} = \text{Total Possible Addresses} - 2\]

Subnetting

🛑 Network Address is assigned by the larger network (e.g. ISP); cannot modify it!

\[\text{Network ID} = \underset{\text{NETWORK ADDRESS}}{\underline{10.22.128.0}} / \underset{\text{CIDR}}{\underline{22}} \rightarrow 10 \text{ host bits}\]

\[\underset{\text{NETWORK ADDRESS}}{\underline{00001010 \cdot 00010110 \cdot 100000}} \space \underset{\text{HOST}}{\underline{00 \cdot 00000000}}\]

💡 Can borrow bits from the host portion to create smaller networks (subnets).

\[\underset{\text{NETWORK ADDRESS}}{\underline{00001010 \cdot 00010110 \cdot 100000}} \space \underset{\text{SUBNET}}{\underline{00 \cdot 0}} \space \underset{\text{HOST}}{\underline{0000000}}\]

If I borrow 3 bits from the host portion

• Subnets: \(2^3 = 8\)
• Hosts per subnet: \(2^{10 - 3} - 2 = 126\)

Subnet Calculations

200.23.20.0/24 = \(\underline{11001000 . 00010111 . 00010100} . 00000000\)

# Host Bits:

# Total:

# Usable:

Broadcast Address:

First Host Address:

Last Host Address:

200.23.20.0/23 = \(\underline{11001000 . 00010111 . 0001010}0 . 00000000\)

# Host Bits:

# Total:

# Usable:

Broadcast Address:

First Host Address:

Last Host Address:

200.23.20.0/25 = \(\underline{11001000 . 00010111 . 00010100 . 0}0000000\)

# Host Bits:

# Total:

# Usable:

Broadcast Address:

First Host Address:

Last Host Address:

Practice

Assigned network address space: 192.168.0.0/23

Assign an appropriate address to each subnet.

Practice

Assigned network address space: 172.18.0.0/23

Assign an appropriate address to each subnet.