Learn Computer Networking PNETLab - Cisco IOS Introduction & Basic Configuration

#Introduction

In episode 1 you understood networking theory as a whole. Now it is time to open the console of your first device in PNETLab and talk directly to Cisco IOS — the operating system that runs Cisco routers and switches. Episode 2 is the gateway to every configuration in this series: without mastering the CLI, there is no VLAN, OSPF, or NAT you can build.

In this episode we discuss the structure of Cisco IOS operation modes, the basic configuration that makes a device secure and manageable, and the verification commands you must memorize. All commands are typed directly on the console of a vIOS Router or IOL Switch node in PNETLab.

#Getting to Know the Cisco Command Line Interface (CLI)

#Structure of Cisco IOS Operation Modes

Cisco IOS divides CLI access into several modes. Each mode has a distinctive prompt that shows your privilege level:

Router>                  -> User EXEC (limited)
Router> enable
Router#                  -> Privileged EXEC
Router# configure terminal
Router(config)#          -> Global Configuration
Router(config)# interface g0/0
Router(config-if)#       -> Interface Configuration

• User EXEC (Router>): the initial mode, limited to informational commands.
• Privileged EXEC (Router#): full access to verification and debugging.
• Global Configuration (Router(config)#): changes the device configuration.
• Interface Configuration (Router(config-if)#): changes the configuration of a single interface.

Navigation between modes is done with enable, configure terminal, and the exit command to go back one level.

#Basic Cisco Router & Switch Configuration

#Naming a Device with hostname

Naming a device makes it easier to identify when working in a large lab. In PNETLab, change the hostname to match the device's role:

enable
configure terminal
hostname R1
enable secret Pn3tLab2026
line console 0
 logging synchronous
 password Cisco123
 login
exit
banner motd # Only network staff are authorized to access this device. #

hostname R1 names the device R1, while enable secret protects Privileged EXEC mode with an encrypted password. In line console 0, you secure access to the console port.

#Configuring Remote SSH and Telnet Access

Remote access is configured in line vty. For security, this series uses SSH and disables Telnet:

configure terminal
hostname R1
ip domain-name pnetlab.local
crypto key generate rsa modulus 2048
username admin secret Cisco123
line vty 0 4
 transport input ssh
 login local
exit

The syntax transport input ssh restricts the vty line to accept only SSH, not Telnet. The command crypto key generate rsa creates the RSA key required for SSH encryption. From any PC you can now log in with ssh admin@<router-interface-ip>.

#Warning Banner Message

banner motd displays a warning message before login. This banner is useful for legal purposes: it asserts that only authorized parties may access the device. The format is banner motd # message # with the # character as the message delimiter.

#Saving the Configuration

All the configuration you type lives in the running-config, which is lost when the device restarts. Save it permanently to the startup-config:

write memory

The command write memory (short for copy running-config startup-config) copies the active configuration to NVRAM. Get into the habit of saving after every configuration task — a habit that will save you in episode 20.

#Initial Status Verification Commands

#show ip interface brief

After configuring, verify the status of all interfaces with the most frequently used verification command in this series:

R1# show ip interface brief
Interface     IP-Address     OK?  Method  Status                  Protocol
GigabitEthernet0/0    192.168.10.1  YES   manual  up                     up
GigabitEthernet0/1    unassigned    YES   unset   administratively down   down

The Status and Protocol columns show the physical and logical layer conditions. If Status is down even though the cable is plugged in, use no shutdown in interface configuration — the most common classic beginner mistake.

#show running-config and show version

show running-config displays the complete active configuration, while show version shows the IOS version, uptime, and device hardware information. These two commands are the first step when auditing an unfamiliar device. Get into the habit of running both every time you enter a new lab in PNETLab.

#Why Basic Configuration Matters

A network device managed remotely without a password is an open door for intruders. enable secret, login local, and transport input ssh are the three minimal layers of protection that must exist on every router and switch before it is connected to a production network. Episodes 15 and 19 will deepen this security with ACLs and AAA.

#Closing

Key takeaways:

• Four main modes: User EXEC, Privileged EXEC, Global Configuration, and Interface Configuration.
• hostname names the device; enable secret protects privileged mode.
• line vty 0 4 with transport input ssh restricts remote access to SSH.
• banner motd displays a warning message before login.
• write memory saves the running-config to the startup-config.
• show ip interface brief is the first verification you should run.

In the next episode, episode 3, we discuss IP addressing — the structure of IPv4 addresses, classful classification, public and private IPs, subnetting with CIDR notation, and Variable Length Subnet Mask (VLSM) calculations. This material will become the basis for subnet allocation in every lab for the rest of the series.