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LPI Linux Essentials Exam 010-160 - Topic 4.4 - IP Addresses, Subnets, NAT & Bridging

Networking is a fundamental aspect of modern computing, connecting devices and allowing them to communicate with each other. In this guide, we'll focus on key networking concepts, starting with IP addresses, which are unique identifiers for each device on a network. We'll explore both IPv4 and IPv6 addresses, their structures, and differences. Subnetting, a technique used to divide a network into smaller, more manageable segments, will also be covered. Additionally, we'll delve into Network Address Translation (NAT) and masquerading, which allow multiple devices on a local network to share a single public IP address. Lastly, we'll touch on bridging, a process that connects two or more network segments, increasing the network's range and allowing devices on different segments to communicate.

IP addresses are essential for the identification and location of devices on a network. IPv4, the most widely used protocol, consists of 32 bits and offers over 4 billion unique addresses. However, due to the limited number of available IPv4 addresses, IPv6 was introduced. IPv6 addresses are 128 bits long, significantly increasing the number of possible addresses. These addresses are expressed in different formats: IPv4 addresses are typically written in decimal notation (e.g.,, while IPv6 addresses use hexadecimal notation. Besides public IP addresses, there are also private IP addresses, reserved for use within a private network. Understanding these concepts is essential for effective network management and troubleshooting.

Understanding IPv4 and IPv6

IPv4 Addresses

IPv4 (Internet Protocol version 4) addresses are 32 bits long, which allows for approximately 4.3 billion unique addresses. They are usually written in decimal format, divided into four octets (8-bit sections), and separated by periods. For example, an IPv4 address looks like, where each number in the sequence can range from 0 to 255.

IPv4 addresses are categorized into different classes, denoted by the first octet, which also indicates the range of addresses and their intended use.

IP Address Classes

IPv4 addresses are categorized into different classes:

  • Class A: Ranges from to The first bit is always zero, allowing for 126 networks and approximately 16,777,214 hosts per network.
  • Class B: Ranges from to, with the first two bits always being 10. It supports 16,384 networks and 65,534 hosts per network.
  • Class C: Ranges from to, designed for small local networks, with about 2,097,150 networks and 254 hosts per network.
  • Class D: Ranges from to, used for multicasting purposes.
  • Class E: Ranges from to, reserved for research.

Private and Public IPv4 Addresses

  • Public IPv4 Addresses are globally unique and used for communication over the internet. They primarily are found within to Classes A, B, and C ranges, excluding the private ranges below.
  • Private IPv4 Addresses are used within private networks and are not routable on the internet. They are in specific ranges reserved for private use:
    • Class A Private Addresses: to
    • Class B Private Addresses: to
    • Class C Private Addresses: to

Understanding these address classes and their distinctions is crucial for effective IP address management and network configuration.

IPv6 Addresses

IPv6 (Internet Protocol version 6) was developed to solve the problem of IPv4 exhaustion. It uses 128-bit addresses, exponentially increasing the number of available addresses. An IPv6 address is expressed in hexadecimal format, divided into eight 16-bit blocks, and separated by colons. For example, an IPv6 address could look like 2001:0db8:85a3:0000:0000:8a2e:0370:7334.

Address Representation in IPv6

  • Zero Compression: In IPv6, consecutive sections of zeros in an address can be compressed using ::. For instance, 2001:0db8:0000:0000:0000:0000:3422:1234 can be written as 2001:db8::3422:1234.
  • Leading Zeroes: Leading zeroes in each block can be omitted for brevity.

Types of IPv6 Addresses

  • Unicast Addresses: Identifies a single interface. A packet sent to a unicast address is delivered to the interface identified by that address.
  • Multicast Addresses: Identifies a group of interfaces. A packet sent to a multicast address is delivered to all interfaces identified by that address.
  • Anycast Addresses: Identifies multiple interfaces. A packet sent to an anycast address is delivered to the nearest interface identified by that address.

Transition from IPv4 to IPv6

As IPv6 adoption increases, there are several transition mechanisms in place for compatibility between IPv4 and IPv6 networks, such as dual-stack, tunneling, and translation techniques. These mechanisms ensure that devices using different versions of the IP protocol can communicate effectively.

Next, we'll explore subnetting, a critical concept for organizing and managing networks efficiently.

Understanding Subnetting in IP Networks

Subnetting is a technique used in networking to break down a large network into smaller, more manageable sections, known as subnets. This method not only improves network performance and security but also optimizes the usage of IP addresses.

A subnet mask is a 32-bit number that helps in segregating an IP address into network and host sections. In a subnet mask, the network bits are set to "1" and the host bits to "0". For instance, a typical subnet mask for a small network is

Understanding Network and Host Portions

In any IP address, there are two parts:

  • The network portion, which identifies the particular network.
  • The host portion, which pinpoints a specific device within that network.

The Process of Subnetting

Subnetting involves splitting a large IP network into smaller networks, increasing efficiency and manageability. This is done by borrowing bits from the host part of an IP address to create new subnets.

Subnetting Example

Let's take an IPv4 address with a subnet mask of or /24. This format implies that 24 bits are used for the network part, leaving 8 bits for the host part. When converted to binary, it's represented as:

  • Network part: 11000000.10101000.00000001
  • Host part: 00000000

If we decide to divide this network into smaller subnets, we would borrow bits from the host part.

Steps in Subnetting

Subnetting involves dividing a larger network into smaller subnets. Here’s a step-by-step breakdown of how this is achieved, using the network with a /24 subnet mask ( as an example.

  1. Borrowing Bits for Subnets: Suppose we borrow 2 bits from the host portion of the IP address. This changes the subnet mask from /24 ( to /26 (, where 26 bits are now dedicated to the network portion.
  2. Determining Subnet Ranges: By borrowing 2 bits, we create 2^2 = 4 subnets. The range of each subnet can be determined by the variation in the borrowed bits. For, the subnets are as follows:
    • Subnet 1:
      • Range: to
      • Network Address: (all borrowed bits are 0)
      • Broadcast Address: (all borrowed bits are 1)
    • Subnet 2:
      • Range: to
      • Network Address:
      • Broadcast Address:
    • Subnet 3:
      • Range: to
      • Network Address:
      • Broadcast Address:
    • Subnet 4:
      • Range: to
      • Network Address:
      • Broadcast Address:
  3. Usable Host Addresses: Each subnet has 2^6 - 2 = 62 usable host addresses. The subtraction of 2 accounts for the network address and the broadcast address in each subnet.

In this example, dividing the network with a /26 subnet mask results in four smaller subnets, each offering 62 usable host addresses. This is an effective strategy for maximizing IP address efficiency in large networks.

Advantages of Subnetting

  • Optimized Network Performance: Subnetting reduces broadcast traffic, thus improving overall network performance.
  • Increased Security: It allows for better isolation and protection of network segments.
  • Efficient Use of IP Addresses: Particularly important in IPv4 networks, subnetting helps in making the most out of the limited IP address space.

This guide aims to simplify the concept of subnetting, making it more understandable for those new to networking.

In the next section, we will explore the concepts of NAT (Network Address Translation) and masquerading, which play crucial roles in how private networks interact with the internet.

Network Address Translation (NAT) and Masquerading

Network Address Translation (NAT) and masquerading are essential techniques used in networking to enable multiple devices on a local network to share a single public IP address. These methods play a crucial role in conserving public IP addresses and enhancing network security.

Network Address Translation (NAT)

NAT is a method that allows multiple devices within a private network to share a single public IP address when communicating with external networks, such as the internet. NAT works by mapping internal private IP addresses to the public IP address when outgoing traffic leaves the private network.

How NAT Works

  • Private IP Addresses: Devices within a local network are assigned private IP addresses. These private addresses are not routable on the public internet.
  • Public IP Address: The NAT device (often a router) has a public IP address that is used for communication with external networks.
  • Translation: When a device from the local network sends data to the internet, the NAT device replaces the source IP address (private) with its own public IP address. This way, responses from external servers are directed back to the NAT device.
  • Port Translation: NAT also includes port translation, allowing multiple devices to share the same public IP address by using different port numbers.


Masquerading is a specific form of NAT, often used in Linux-based systems. It is also known as Network Address Port Translation (NAPT) or IP Masquerading. Masquerading allows multiple devices with private IP addresses to access the internet using a single public IP address.

Advantages of Masquerading

  • Simplified Configuration: Masquerading simplifies the configuration of NAT by dynamically mapping internal IP addresses to the public IP address.
  • Increased Security: It enhances security by hiding the internal network structure from external networks.
  • Effective Resource Utilization: Masquerading efficiently uses a single public IP address for multiple devices.

NAT and Masquerading in Linux

In Linux, NAT and masquerading are achieved using tools like iptables and nftables. These tools allow administrators to configure the network to perform NAT and masquerading functions, ensuring that devices on the local network can access the internet seamlessly while sharing a single public IP address.

In the next section, we will explore the concept of bridging in networking, which involves connecting two or more network segments to expand the network's reach.

Bridging in Networking

Bridging is a networking technique that allows two or more separate network segments to function as a single network. It is a method of connecting and expanding the reach of a network, enabling devices on different segments to communicate with each other seamlessly.

Understanding Bridging

  • Bridge Device: A bridge device is used to connect two or more network segments. It operates at the data link layer (Layer 2) of the OSI model.
  • Transparent Bridge: A transparent bridge is a common type of bridge that operates without the need for any IP configuration. It learns the MAC addresses of devices connected to its segments and uses this information to make forwarding decisions.
  • Spanning Tree Protocol (STP): Bridges often use the Spanning Tree Protocol to prevent loops in bridged networks, ensuring efficient and safe data transfer.

Benefits of Bridging

  • Network Expansion: Bridging extends the reach of a network, allowing devices on different segments to communicate as if they were on the same segment.
  • Segment Isolation: Bridging can be used to isolate segments, which can enhance security and reduce broadcast traffic.
  • Fault Tolerance: Spanning Tree Protocol (STP) in bridging provides fault tolerance by preventing network loops.

Practical Use Cases

  • Connecting LANs: Bridging is commonly used to connect LANs in different physical locations, creating a unified network.
  • Virtual LANs (VLANs): In larger networks, VLANs can be created using bridges to logically separate different parts of the network.
  • Wireless Access Points: Many wireless access points have bridge capabilities to connect wireless and wired networks.


In summary, this guide has provided a foundational understanding of key networking concepts. We began by exploring IPv4 and IPv6 addressing, delving into their structures, representations, and purposes. Subnetting, a technique for efficient network organization, was explained, emphasizing its benefits in managing IP address allocation.

We then discussed Network Address Translation (NAT) and masquerading, vital for allowing multiple devices to share a single public IP address, facilitating internet connectivity in private networks. Lastly, we examined bridging, a method used to connect and expand network segments, enhancing communication between devices on different segments.

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