Days 7, 8: IPv4

IPv4 Addressing

Overview

IPv4 Fundamentals

IPv4 (Internet Protocol Version 4) is the primary Layer 3 protocol used in modern networks. It provides:

• Logical addressing via 32-bit IP addresses

• Connectivity between different networks (requires routers)

• Path selection between source and destination hosts

Key Distinction from Layer 2:

• Layer 2 (Switches) expands networks using MAC addressing within the same LAN

• Layer 3 (Routers) separates and connects different networks using IP addressing

 

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IP Address Structure

The IPv4 Address Format

IPv4 addresses consist of 32 bits divided into 4 octets (8 bits each), written in Dotted Decimal Notation:

192.168.1.254

Each decimal number = 8-bit octet

Each octet can range from 0 to 255

Binary Representation:

192        .    168        .    1          .    254

11000000   .    10101000   .    00000001   .    11111110

Network and Host Portions

An IP address is split into two parts:

1. Network Portion — Identifies the network itself

2. Host Portion — Identifies the device within that network

Notation: CIDR (Classless Inter-Domain Routing)

The slash notation /X indicates how many bits are the network portion:

192.168.1.254/24

/ 24 = The first 24 bits are the NETWORK portion

The remaining 8 bits are the HOST portion

NETWORK: 192.168.1   (first 3 octets)

HOST:    .254        (last octet)

All devices on the same network must have the same network portion but different host portions:

192.168.1.100   ← Same network (192.168.1)

192.168.1.105   ← Same network (192.168.1)

192.168.1.205   ← Same network (192.168.1)

Network Boundaries

Routers separate networks. When a router is placed between two networks:

Network 1: 192.168.1.0/24

Router Interface 1 (G0/0): 192.168.1.254

Router Interface 2 (G0/1): 192.168.2.254

Network 2: 192.168.2.0/24

Each router interface gets a unique IP address on the network it connects to.

Broadcasts do not cross routers: When a broadcast packet hits a router, it stops. Broadcasts stay within the local LAN.

 

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Number Systems for IP Addressing

Converting Binary to Decimal

Each bit position represents a power of 2:

Bit positions:  128  64  32  16  8  4  2  1

Example:        1    0   0   1   1  1  1  1  = 10011111

Calculation:

1 × 128 = 128

0 × 64  = 0

0 × 32  = 0

1 × 16  = 16

1 × 8   = 8

1 × 4   = 4

1 × 2   = 2

1 × 1   = 1

Sum: 128 + 16 + 8 + 4 + 2 + 1 = 159

Another Example: 01110110

64 + 32 + 16 + 4 + 2 = 118

Converting Decimal to Binary

Method 1: Subtraction (Left to Right)

Convert 221 to binary:

221 - 128 = 93  ✓ → write 1 in 128 position

93  - 64  = 29  ✓ → write 1 in 64 position

29  - 32  = X   ✗ → write 0 in 32 position

29  - 16  = 13  ✓ → write 1 in 16 position

13  - 8   = 5   ✓ → write 1 in 8 position

5   - 4   = 1   ✓ → write 1 in 4 position

1   - 2   = X   ✗ → write 0 in 2 position

1   - 1   = 0   ✓ → write 1 in 1 position

Result: 11011101

Method 2: Subtraction from 255

Convert 207 to binary:

255 - 207 = 48

Now find where 48 appears in binary positions:

48 = 32 + 16

So the 0's are in positions 32 and 16

The 1's are everywhere else

Result: 11001111

Convert Binary IP to Decimal

Binary: 10011010.01001110.01101111.00100000

Octet 1: 128 + 16 + 8 + 2 = 154

Octet 2: 64 + 8 + 4 + 2 = 78

Octet 3: 64 + 32 + 8 + 4 + 2 + 1 = 111

Octet 4: 32

IPv4 Address: 154.78.111.32

 

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IPv4 Address Classes

IPv4 addresses are divided into 5 classes based on the first octet:

For CCNA, focus on Classes A, B, and C.

Class A (0–126)

Example: 10.0.0.0/8

Range:   10.0.0.1 to 10.255.255.254

Hosts:   2^24 - 2 = 16,777,214

Network portion: First octet only

Host portion: Last 3 octets

• Very large networks

• Rarely used in modern networks (reserved for special use)

Class B (128–191)

Example: 172.16.0.0/16

Range:   172.16.0.1 to 172.16.255.254

Hosts:   2^16 - 2 = 65,534

Network portion: First 2 octets

Host portion: Last 2 octets

• Medium-sized networks

• Common in larger enterprises

Class C (192–223)

Example: 192.168.1.0/24

Range:   192.168.1.1 to 192.168.1.254

Hosts:   2^8 - 2 = 254

Network portion: First 3 octets

Host portion: Last octet

• Smaller networks

• Most common in small to medium businesses

Special Addresses: Loopback (Class A)

127.0.0.0 to 127.255.255.255 — Reserved for loopback testing

127.0.0.1 is the standard loopback address

Used to test the network stack (OSI & TCP/IP model) on a local device

Does not go to the network; stays on the local machine

Useful for testing TCP/IP implementation without actual network traffic

 

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Subnet Masks and Network Addresses

Subnet Mask (Netmask)

A subnet mask identifies which bits are the network portion and which are the host portion:

• 1's in the binary netmask = network bits

• 0's in the binary netmask = host bits

Standard Class Netmasks:

Example: Identifying Network and Host with Mask

IP Address:    192.168.1.100

Netmask (/24): 255.255.255.0

Match the 1's in the netmask:

Network:       192.168.1.0

Host portion:  .100

 

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Network Addresses and Broadcast Addresses

Network Address (First Address)

When the host portion is all 0's, it's the network address — the identifier of the network itself.

192.168.1.0/24 = NETWORK ADDRESS (not assigned to any host)

Broadcast Address (Last Address)

When the host portion is all 1's, it's the broadcast address — used to send data to all hosts on the network.

192.168.1.255/24 = BROADCAST ADDRESS (not assigned to any host)

Destination IP:  192.168.1.255

Destination MAC: FFFF.FFFF.FFFF

First and Last Usable Addresses

Because network and broadcast addresses are reserved, the usable host range is:

Network Address:       192.168.1.0   ← Not usable

First Usable Address:  192.168.1.1   ← Usable

...

Last Usable Address:   192.168.1.254 ← Usable

Broadcast Address:     192.168.1.255 ← Not usable

 

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Calculating Maximum Hosts per Network

Formula: 2^N - 2

Where N = number of host bits

(The "-2" accounts for the network address and broadcast address)

Class A Example

10.0.0.0/8

Host bits: 24 (since /8 network bits)

Maximum hosts: 2^24 - 2 = 16,777,216 - 2 = 16,777,214

Class B Example

172.16.0.0/16

Host bits: 16

Maximum hosts: 2^16 - 2 = 65,536 - 2 = 65,534

Class C Example

192.168.1.0/24

Host bits: 8

Maximum hosts: 2^8 - 2 = 256 - 2 = 254

 

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IPv4 Header (Layer 3)

Header Overview

The IPv4 header contains many more fields than the Ethernet header:

Minimum Header Length: 20 bytes Maximum Header Length: 60 bytes

Key Header Fields

Source IP Address (32 bits)

• IP address of the sender/originating device

Destination IP Address (32 bits)

• IP address of the intended receiver/destination

Total Length (16 bits)

• Indicates the total length of the packet (header + data)

• Measured in bytes (not 4-byte increments like IHL)

• Minimum: 20 bytes (header only, no data)

• Maximum: 65,535 bytes (2^16 - 1)

Header Checksum (16 bits)

• Calculated checksum for error detection

• Routers calculate this and drop the packet if checksums don't match

• Only checks the IPv4 header, not the encapsulated data

• The encapsulated protocol (TCP/UDP) handles data error checking

Time To Live (TTL) (8 bits)

• Prevents packets from circulating indefinitely

• Decremented by 1 at each router

• When TTL reaches 0, the packet is dropped

• Default value: typically 64 or 255

Version (4 bits)

• IP version: 4 for IPv4, 6 for IPv6

 

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Cisco Router Interface Configuration

Viewing Interface Status

Router# show ip interface brief

Output columns:

Status Meanings:

• up — Interface is operational

• down — Interface has a problem or no cable

• administratively down — Interface has been disabled with shutdown command

Default Behavior:

• Router interfaces are administratively down by DEFAULT

• Switch interfaces are NOT administratively down by default

Entering Interface Configuration Mode

Router# configure terminal

Router(config)# interface gigabitethernet 0/0

Shorthand can be used: g0/0, fa0/0, etc.

Assigning an IP Address

Router(config-if)# ip address 10.255.255.254 255.0.0.0

Format: ip address <IP> <netmask>

Enabling the Interface

Router(config-if)# no shutdown

This removes the "administratively down" status.

Expected output:

Interface GigabitEthernet0/0 changed state to up

Line protocol on Interface GigabitEthernet0/0, changed state to up

Verifying Configuration

From interface configuration mode, use do to execute privileged commands:

Router(config-if)# do show ip interface brief

This confirms the interface is up and the IP address is assigned correctly.

Detailed Interface Information

Router# show interfaces gigabitethernet 0/0

Shows extensive Layer 1, Layer 2, and Layer 3 information:

• MAC address (BIA)

• IP address

• Interface encapsulation

• MTU size

• Error statistics

• And much more

Router# show interfaces description

Displays descriptions assigned to each interface (if any).

 

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Summary: Common IPv4 Classes at a Glance

 

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Quick Conversion Reference

Common Decimal to Binary for Octets:

 

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Key Takeaways for CCNA

✅ IPv4 addresses are 32-bit logical addresses, split into network and host portions

✅ The CIDR notation (/8, /16, /24) indicates the network portion length

✅ Routers separate networks; switches expand networks within the same LAN

✅ IPv4 Classes A, B, C differ in network size (more hosts = more network bits needed)

✅ Network addresses (all host bits = 0) and broadcast addresses (all host bits = 1) are reserved and not usable

✅ Usable host range = 2^(host bits) - 2

✅ Router interfaces must be enabled with no shutdown (administratively down by default)

✅ Subnet masks use 1's for network bits and 0's for host bits

✅ Binary-to-decimal conversion is crucial for subnetting (coming next!)

✅ Broadcasts don't cross routers — they stay within the local LAN

✅ Verify interface configuration with show ip interface brief and show interfaces <interface>