Days 7, 8: IPv4

CCNA 200-301 Study Guide: IPv4 and Core Router ConfigurationAddressing

1.0 IntroductionOverview

ThisIPv4 guideFundamentals

IPv4 (Internet Protocol Version 4) is the primary Layer 3 fundamentals,protocol IPv4used addressing,in andmodern corenetworks. routerIt configuration. These topics align with the Network Fundamentals (20%) and IP Connectivity (25%) domains of the CCNA 200-301 exam.provides:

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

• PathLogical Selection:addressing Usesvia a32-bit RoutingIP Table to determine the most efficient path for packets.addresses

• BroadcastConnectivity Domainbetween Boundary:different Routersnetworks do(requires not forward broadcasts by default. Each interface is a separate broadcast domain.routers)

• InterfacePath Defaultselection State:between Unlikesource switches,and Ciscodestination router interfaces are administratively down by default. They require the no shutdown command to function.hosts

3.0

Key DeconstructingDistinction 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

IP Address Structure

The IPv4 Address Format

An IPv4 addressaddresses isconsist aof 32-bit32 logicalbits identifierdivided representedinto 4 octets (8 bits each), written in Dotted Decimal NotationNotation:

192.168.1.1).254

3.

Each decimal number = 8-bit octet

Each octet can range from 0 to 255

Binary Representation:

192        .    168        .    1 Structure         .    254

11000000   .    10101000   .    00000001   .    11111110

Network and Host Portions

An IP address is split into two parts:

1. Network Portion:Portion — Identifies the specificnetwork subnet.itself

2. Host Portion:Portion — Identifies the specific device onwithin that subnet.network

3.2

Notation: BinaryCIDR Foundations(Classless Inter-Domain Routing)

The octetslash (8notation bits)/X servesindicates ashow many bits are the fundamentalnetwork buildingportion:

192.168.1.254/24

/ addressing.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.

Number Systems for IP Addressing

Converting Binary to Decimal

Each bit position within an octet corresponds torepresents a specific 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 ($2^n$),Left decreasingto 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 left255

Convert 207 to right.binary:

Positional255 Values- Table207 = 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

IPv4 Address Classes

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

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.

5.0

For PrivateCCNA, vs.focus Publicon AddressingClasses (RFCA, 1918)

Private addresses are used internallyB, and are not routable on the public internet.C.

5.Class A (0–126)

Example: 10.0.0.0/8

Range:   10.0.0.1 RFCto 191810.255.255.254

Hosts:   Ranges2^24 - 2 = 16,777,214

Network portion: First octet only

Host portion: Last 3 octets

• ClassVery A:large 10.0.0.0 – 10.255.255.255networks

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

Class B:B (128–191)

Example: 172.16.0.00/16

Range:   172.31.16.0.1 to 172.16.255.255254

Hosts:   2^16 - 2 = 65,534

Network portion: First 2 octets

Host portion: Last 2 octets

Medium-sized networks

ClassCommon C:in 192.168.0.0larger – 192.168.255.255enterprises

5.Class C (192–223)

Example: 192.168.1.0/24

Range:   192.168.1.1 to 192.168.1.254

Hosts:   2^8 - 2 Special= Reserved254

Network portion: First 3 octets

Host portion: Last octet

• Loopback:Smaller 127.0.0.1 (Tests the local TCP/IP stack).networks

• APIPA:Most 169.254.0.0/16common (Self-assignedin whensmall DHCPto fails).medium businesses

6.

Special Addresses: Loopback (Class A)

127.0.0.0 Essentialto 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

Subnet Masks and Network CalculationsAddresses

6.1Subnet ReservedMask Addresses in a Subnet(Netmask)

A

Networkmask Address:identifies The first address (all hostwhich bits are 0).the Identifiesnetwork portion and which are the subnet.host portion:

1's in the binary netmask = network bits

Broadcast0's Address:in Thethe lastbinary addressnetmask (all= 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

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 communicateany withhost)

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 subnet.

6.2 Calculating Usable Hosts

To find the number of usable host IPs:network.

$$2^h192.168.1.255/24 -= 2$$BROADCAST ADDRESS (not assigned to any host)

(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>Destination enableIP:                                        # 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/Mask255

Router(config-if)#Destination descriptionMAC: 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 NVRAMFFFF.FFFF.FFFF

7.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

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 CLI= Shortcuts16,777,216 - 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

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)

• do:IP Executeaddress EXECof commandsthe fromsender/originating configdevice

  modes

Destination IP Address (e.g.,32 dobits)

IP brief).address of the intended receiver/destination

Total Length (16 bits)

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

no:Measured Negatesin a commandbytes (e.g.,not 4-byte increments like IHL)

Minimum: 20 bytes (header only, no shutdowndata)

Maximum: ip65,535 addressbytes to(2^16 remove).- 1)

8.0

Header VerificationChecksum (16 bits)

Calculated checksum for error detection

Routers calculate this and Troubleshootingdrop 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

Cisco Router Interface Configuration

Viewing Interface Status

Router# show ip interface brief

Output columns:

9.0

Status Key Takeaways for the ExamMeanings:

• Binary:up You— mustInterface beis able to convert between decimal and binary quickly.operational

• Defaultdown Off:— AlwaysInterface rememberhas a problem or no shutdown for routers.cable

• Volatileadministratively Memory:down Configurations— inInterface RAMhas been disabled with shutdown command

Default Behavior:

Router interfaces are lostadministratively ondown reboot;by always copy run start.DEFAULT

Subnetting:Switch Neverinterfaces assignare theNOT Networkadministratively ordown Broadcastby addresses to a host.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).

Summary: Common IPv4 Classes at a Glance

Quick Conversion Reference

Common Decimal to Binary for Octets:

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>