Networking and Firewalls

IPv4 Addressing

IPv4 (Internet Protocol version 4) is a numerical label assigned to devices connected to a network for identification and communication. It uses a 32-bit address, written in dotted decimal notation (e.g., 192.168.1.1).

IP Address Classes:

Netmask & Subnet Mask

• Netmask: A 32-bit mask used to divide an IP address into network and host portions.

• Subnet Mask: A special netmask that helps in defining subnets within a network. Example: 255.255.255.0 indicates that the first 3 octets define the network, and the last octet defines the host.

Gateway

A gateway is a device (typically a router or firewall) that connects different networks, allowing communication between them. In an industry setting, a gateway can be a router or the IP of a firewall.

CIDR (Classless Inter-Domain Routing) Range

CIDR notation is a method for allocating IP addresses and routing.

• Format: IP address / Subnet Bits

• Example: 192.168.1.0/24 (24 bits for the network, 8 bits for hosts).

Network ID & Broadcast ID

• Network ID: The first address in the subnet (e.g., 192.168.20.0/21).

• Broadcast ID: The last address in the subnet (e.g., 192.168.20.255).

• Network Range: The range of usable IPs (e.g., 192.168.20.1 - 192.168.20.254).

• Total IPs Possible: 2^Host Bits - 2 (subtracting network and broadcast addresses).

Example Calculation:

• Given: 192.168.20.15, Netmask: 255.255.255.0

• Network ID: 192.168.20.0/21

• Broadcast ID: 192.168.20.255

• Network Range: 192.168.20.1 - 192.168.20.254

• Total Number of IPs: 2^(32 - subnet bits) - 2

• Host Bits: 32 - 24 = 8

Firewall & Windows Defender Firewall

A firewall is a security system that monitors and controls incoming/outgoing traffic based on security rules.

• Explicitly Allow: Grants access to specific rules.

• Explicitly Deny: Blocks access regardless of other rules.

• Deny All: Blocks all traffic unless explicitly allowed.

• Whitelist: A strict policy where only explicitly allowed traffic is permitted.

• First IP Used as Gateway: The first usable IP in a subnet is typically reserved for the gateway.

• Only IP for External Access: The gateway is the only IP allowing outbound traffic.

Network Devices: Switch

A network switch connects devices within a LAN (Local Area Network) and directs data packets only to the intended device.

Linux Networking & Package Management Commands

APT (Advanced Package Tool)

APT is the package manager for Debian-based Linux distributions like Ubuntu, used to install, update, and remove software.

Debian-Based Linux

Debian-based Linux distributions are operating systems built on the Debian Linux framework. They use the APT package manager and follow Debian’s software repository structure. Examples include:

• Ubuntu

• Kali Linux

• Linux Mint

• Parrot OS These distributions are known for their stability, extensive software support, and strong community backing.

Communication in the Same Network

Devices within the same subnet can communicate without a router. If they are in different subnets, they need a gateway.

Linux Commands Explained

1. sudo apt update

• This command updates the package lists for APT (Advanced Package Tool) to ensure you get the latest available versions of software and security patches.
• It does not upgrade or install any new packages but refreshes the package repository metadata.
• Example Output:
  Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB] Reading package lists... Done

2. sudo apt install apache2

• Installs the Apache2 web server package on a Debian-based Linux system.

• The sudo command ensures that the installation runs with administrative privileges.

• Example Usage:
  sudo apt install apache2

• During installation, it may prompt for confirmation (Y/n).

3. systemctl status apache2

• Checks the status of the Apache2 service.
• It provides information on whether the service is active (running), inactive, or failed.
• Example Output:
  apache2.service - The Apache HTTP Server Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled) Active: active (running) since Mon 2024-02-20 12:34:56 UTC; 5min ago

4. sudo systemctl start apache2

• Starts the Apache2 web server service.
• If Apache is installed but not running, this command ensures it is up and running.
• Example Usage:
  sudo systemctl start apache2

5. apt search apache2

• Searches the available package repositories for Apache2 and related packages.
• Helps find package names and descriptions before installation.
• Example Output:
  apache2/focal-updates,focal-security 2.4.41-4ubuntu3.9 amd64 Apache HTTP Server

6. sudo -s

• Opens a root shell, giving full administrative access.
• Equivalent to switching to the root user (su) but uses sudo.
• Example Usage:
  sudo -s root@hostname:/home/user#

7. whoami

• Displays the current logged-in username.
• Example Output:
  user1

8. sudo whoami

• Runs whoami with root privileges, returning root if executed successfully.
• Example Output:
  root

9. rm filename

• Deletes the specified file.

• Example Usage:
  rm myfile.txt

• Warning: There is no undo option; deleted files cannot be recovered easily.

• To force deletion without confirmation:
  rm -f filename

• To remove a directory and all its contents:
  rm -r directory_name

Note:

• apt : package manager
• systemctl : service manager
• enable : service is there in the system startup

Difference Between enable and start

*Homework:

• Study IPv4 and Ethernet
• Review the OSI Model

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