Days 7, 8: IPv4

CCNA 200-301 Study Guide: IPv4 and Core Router Configuration
1.0 Introduction
This guide covers Layer 3 fundamentals, IPv4 addressing, and core router configuration. These topics align with the Network Fundamentals (20%) and IP Connectivity (25%) domains of the CCNA 200-301 exam.
2.0 Layer 3 Fundamentals: The Role of the Router
The Network Layer (Layer 3) provides logical addressing and path determination across different network segments.
Key Characteristics of a Router
Path Selection: Uses a Routing Table to determine the most efficient path for packets.
Broadcast Domain Boundary: Routers do not forward broadcasts by default. Each interface is a separate broadcast domain.
Interface Default State: Unlike switches, Cisco router interfaces are administratively down by default. They require the no shutdown command to function.
3.0 Deconstructing the IPv4 Address
An IPv4 address is a 32-bit logical identifier represented in Dotted Decimal Notation (e.g., 192.168.1.1).
3.1 Structure
Network Portion: Identifies the specific subnet.
Host Portion: Identifies the specific device on that subnet.
3.2 Binary Foundations
The octet (8 bits) serves as the fundamental building block of IPv4 addressing. Each bit position within an octet corresponds to a specific power of 2 ($2^n$), decreasing from left to right.
Positional Values Table
Bit Position
7
6
5
4
3
2
1
0
Power of 2
2^7
2^6
2^5
2^4
2^3
2^2
2^1
2^0
Decimal Value
128
64
32
16
8
4
2
1
TL;DR: An octet represents values from 0 to 255. To calculate a decimal value, sum the decimal weights of every bit set to "1".
Examples:
192 in binary: 11000000 ($128 + 64$)
255 in binary: 11111111 (All bits on)
4.0 IPv4 Address Classification (Classful System)
While modern networks use CIDR (Classless), understanding the original classes is essential for the exam.
Class
Range (1st Octet)
Default Mask
Hosts per Network
Use Case
A
1 – 126
255.0.0.0 (/8)
16,777,214
Very Large Networks
B
128 – 191
255.255.0.0 (/16)
65,534
Medium/Large Networks
C
192 – 223
255.255.255.0 (/24)
254
Small Networks
D
224 – 239
N/A
N/A
Multicast
E
240 – 255
N/A
N/A
Experimental
5.0 Private vs. Public Addressing (RFC 1918)
Private addresses are used internally and are not routable on the public internet.
5.1 RFC 1918 Private Ranges
Class A: 10.0.0.0 – 10.255.255.255
Class B: 172.16.0.0 – 172.31.255.255
Class C: 192.168.0.0 – 192.168.255.255
5.2 Special Reserved Ranges
Loopback: 127.0.0.1 (Tests the local TCP/IP stack).
APIPA: 169.254.0.0/16 (Self-assigned when DHCP fails).
6.0 Essential Network Calculations
6.1 Reserved Addresses in a Subnet
Network Address: The first address (all host bits are 0). Identifies the subnet.
Broadcast Address: The last address (all host bits are 1). Used to communicate with all hosts on the subnet.
6.2 Calculating Usable Hosts
To find the number of usable host IPs:
$$2^h - 2$$
(Where $h$ is the number of host bits. The $-2$ accounts for the Network and Broadcast addresses.)
7.0 Core Cisco IOS Router Configuration
7.1 Basic Configuration Workflow
Router> enable                                        # Enter Privileged EXEC mode
Router# configure terminal                            # Enter Global Configuration mode
Router(config)# interface GigabitEthernet0/0/0        # Enter Interface mode
Router(config-if)# ip address 192.168.1.1 255.255.255.0 # Assign IP/Mask
Router(config-if)# description Link to LAN-SWITCH-01   # Optional Label
Router(config-if)# no shutdown                         # Enable the interface
Router(config-if)# end                                 # Exit to Privileged EXEC
Router# copy running-config startup-config            # Save to NVRAM
7.2 CLI Shortcuts
do: Execute EXEC commands from config modes (e.g., do show ip int brief).
no: Negates a command (e.g., no shutdown to enable, no ip address to remove).
8.0 Verification and Troubleshooting
Command
Purpose
show ip interface brief
Concise summary of IP, status (Up/Down), and Protocol.
show interfaces
Detailed stats, error counters (CRC, Collisions), and Layer 1/2 health.
show running-config
View the current active config in RAM.
ping
Tests end-to-end Layer 3 connectivity.
9.0 Key Takeaways for the Exam
Binary: You must be able to convert between decimal and binary quickly.
Default Off: Always remember no shutdown for routers.
Volatile Memory: Configurations in RAM are lost on reboot; always copy run start.
Subnetting: Never assign the Network or Broadcast addresses to a host.