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Securing your local network

Better understand your home network

Improve Your Personal Digital Security

Better understand your home network

  • Internet
  • The role of the modem and router
  • The local area network (LAN)
  • Wi-Fi
  • Local IP addresses
  • Network ports
Digital security often starts with a good command of the home network. Your home network is the Internet's first point of entry into your personal environment, and it's the Internet that poses the biggest threat to both your security and your privacy.
Understanding how your local network works, how your data flows and which devices are responsible for managing these flows is vital to your digital sovereignty.
In this chapter, we'll gradually explore the basic concepts of home networking, from an explanation of what the Internet is to concrete details such as the allocation of local IP addresses and the role of the router. The aim is to go back over the theoretical basics that are essential for a proper understanding of the following chapter, which will deal with more practical aspects.

Internet

A brief history of the Internet

The history of the Internet goes back to the late 1960s, in a context of scientific and military research. The initial project, dubbed "ARPANET", was launched by the American agency ARPA (now DARPA) to interconnect several research centers and enable the sharing of resources.
The aim was also to design a resilient network, capable of continuing to operate even if part of its nodes were destroyed (a necessity in the midst of the Cold War). ARPANET enabled the first communications between remote computers in 1969.
Over the decades, the project expanded, notably with the standardization of the TCP/IP protocols in 1983, which became the common technical foundation of the entire Internet. In the 1990s, the advent of the World Wide Web, discussed in a previous chapter, made the Internet accessible to the general public by simplifying access to documents via browsers. This major turning point transformed the Internet into a universal communication and information tool.

What is the Internet?

The Internet, which literally means "interconnected network", is a global infrastructure that enables billions of devices to communicate with each other. It's not a single service, nor a specific location, but rather a collection of public and private computer networks, interconnected on a planetary scale. Each player (private individual, company, public authority, etc.) can connect its own network to the rest of the world, provided it complies with certain standardized technical protocols that enable devices to understand each other.
The Internet infrastructure is based mainly on two protocols:
  • IP (Internet Protocol) Identifies each connected device and routes data packets.
  • TCP (Transmission Control Protocol) Segments the flow, checks for errors and puts the segments back in order.
→ For real-time uses where reliability is less important than latency (VoIP, DNS, streaming, etc.), another transport protocol is often used instead of TCP: UDP (User Datagram Protocol).
These two protocols work together to ensure that messages sent from one point to another arrive complete and in the right order.
In concrete terms, when you visit a website, for example by typing an address into your browser, several operations are triggered in the background. First, your computer or smartphone must convert the readable address (such as planb.network) into an IP address, using a service called DNS (Domain Name System). Once the web server's IP address has been obtained, the data can be sent to its destination.
This data then follows a complex path. First, it passes through your home router and modem, before reaching your ISP's infrastructure. From there, it potentially crosses several intermediate networks (sometimes transoceanic cables, transit routers, Internet exchange points, etc.) before reaching its destination: The server of the site you wish to visit. This in turn sends the data (the site's content) in the opposite direction.
The Internet can therefore be summed up as an immense decentralized network, based on common rules, where each device can potentially communicate with another, wherever it may be in the world. But while this openness enables digital exchanges, it also requires you to adopt a vigilant posture: Every online action passes through infrastructures that you have no control over, and the confidentiality of your exchanges depends on how you configure and use these tools.

The role of the modem and router

In most modern homes, Internet access is provided via a central device that acts as a modem and router. The name of this device varies from country to country (in France, it's called an "Internet Box"). It's this multi-functional device supplied by the ISP that connects you to the rest of the Internet network.
→ ISP = Internet Service Provider.
This device combines 2 main functions:
  • The modem role
The modem (short for "modulator-demodulator") is the component that connects your ISP network to your home. It adapts the electrical or optical signals received (via fiber optics, coaxial cable, ADSL or the mobile network) into digital signals that can be understood by your devices. Conversely, it converts the data from your equipment into a format that can be transmitted over the operator's infrastructure. This step is essential if your home network is to communicate with the Internet.
  • The router role
The router is the heart of your local network. It distributes the Internet connection to all your devices: Computers, smartphones, connected objects, printers, etc. It assigns each of them a local IP address (Distinct from your public IP visible on the Internet). The router then organizes incoming and outgoing traffic, ensuring that data reaches the right device. It can also integrate a firewall, filter network packets or manage port forwarding rules. NAT masks your local network from the eyes of the Internet, using a single public IP address to represent all your devices. Only the router knows which local device initiated which request.
In some countries, these two functions are sometimes split between two separate devices: A modem and a router.
In practice, a modern modem router often includes other complementary functions: A Wi-Fi access point, an Ethernet switch, an administration interface, and sometimes even a file server and other features.
Understanding the role of the router-modem is a key factor in securing your local network. In a way, this device forms the boundary between the outside world (the Internet) and your private digital environment. Incorrect configuration, weak passwords or obsolete firmware can turn this gateway into a gaping hole. Conversely, a well-configured modem-router becomes a bulwark against intrusions.
In the next chapter, we'll look at how to optimize this configuration, identify common weak points, and regain control of this device.

The local area network (LAN)

The local network, or "LAN" (Local Area Network), refers to all the digital connections inside your home. It includes all devices connected to your router: Computers, smartphones, tablets, printers, etc.
Unlike the Internet, which is a global, public network, the local network is a private, partly isolated space designed to enable devices to communicate with each other, without their exchanges leaving your home. For example, when you send a file from your computer to a Wi-Fi-connected printer, or control a connected light bulb via your telephone, these communications take place exclusively locally, without passing through a remote server.
This is based on a set of standard technologies and protocols, the most important of which is the IP protocol. Each device on your local network is assigned a local IP address (usually in the format 192.168.x.x), which serves to uniquely identify it within that network. This address is not visible from the Internet. Only your public IP address, shared by all devices via the modem, is exposed outside your home.
But this apparent isolation should not give you a false sense of security. A poorly configured local network can become a gateway for attacks, particularly in the following cases:
  • If open services (such as file shares or administration interfaces) are accessible from the Internet
  • If Wi-Fi is not secure (weak password or obsolete encryption system)
  • If certain connected objects communicate with the outside world in cleartext or leave ports open
So it's important to understand your network's internal flows, and control which devices can interact, both with each other and with the outside world.

Wi-Fi

Wi-Fi (short for "Wireless Fidelity") is a wireless communication technology that lets you connect your devices to your local network, without the need for a cable. This technology is based on the transmission of data via radio waves, generally in the 2.4 GHz and 5 GHz frequency bands (or even 6 GHz for Wi-Fi 6E and Wi-Fi 7 networks).
But this ease of use comes with its own risks. Unlike wired connections (such as Ethernet), Wi-Fi signals penetrate walls and can be picked up from dozens of meters away. This means that people outside your home (on the street, in a neighboring building, even from a vehicle parked nearby) can try to detect your network and connect to it if it's not properly secured.
An attacker with access to your Wi-Fi network could then:
  • Intercept unencrypted data passing over the network (e.g. DNS requests or HTTP traffic)
  • Access your files or devices if they are poorly protected
  • Inject malicious content into your browser
  • Use your Internet connection without your knowledge, or even carry out online attacks from your public IP address
To guard against these threats, you need to secure your Wi-Fi network. We'll look at how to do this in the next few chapters.

Local IP addresses

When you connect a device to your home network, it receives a local IP address. This address is made up of four numbers separated by dots (e.g. 192.168.1.14). It is used to uniquely identify each device within your local network. It's thanks to this address that your router knows which device to send data to (for example, a web page or video you've requested).
It's important to understand that these local IP addresses are not visible from the Internet. They belong to reserved ranges, defined by international standards, which cannot be used on the public network. The three most common local IP address ranges are:
  • 192.168.0.0 to 192.168.255.255 (generally for private users)
  • 10.0.0.0 to 10.255.255.255 (generally for companies)
  • 172.16.0.0 to 172.31.255.255
The same device can therefore have an IP address like 192.168.1.42 at home, while another user in another country will have another device with the same IP address on his own local network, without there ever being any conflict, because these addresses don't leave the home network. It's the router that provides the link between your home network and the public Internet, translating these addresses via a mechanism called "NAT" (Network Address Translation).
Local IP addresses are generally assigned dynamically, using DHCP (Dynamic Host Configuration Protocol). When you connect a new device to the network, it sends a DHCP request for an address, and the router assigns it an available free address within the defined range. This address may vary over time.
It is, however, possible to configure fixed IP addresses for certain devices. This means reserving a specific address for a given device, on a permanent basis. This is useful in a number of cases, for example:
  • For easy access to a personal server or NAS from another device
  • To print remotely to a network printer without having to search for its address
  • To create filtering or port forwarding rules in your router

Network ports

To understand how a local network works, you need to know that each IP address (whether local or public) is associated with a series of numerical ports, numbered from 0 to 65535. These ports are not physical: They are virtual entry or exit points that enable applications and services to communicate across the network.
Every time a device communicates via the Internet or a local network, it does so by combining an IP address and a port number. This makes it possible to identify not only which device to send data to, but also which specific application or service on that device. For example:
  • Port 80 is used to access HTTP web pages
  • Port 443 is used to access HTTPS web pages (encrypted connection)
  • Port 22 is used for SSH connections
  • Ports 25, 465 and 587 are used for sending e-mails (SMTP)
  • Port 110 is used to retrieve e-mails via the POP3 protocol
  • Port 143 is used to retrieve e-mails via the IMAP protocol
  • Port 21 is used for file transfers via the FTP protocol
  • Port 53 is used for DNS queries...
These ports are also used in systems such as Bitcoin or Tor, where specific ports are used by default for peer-to-peer communications or traffic routing:
  • Port 8333 is used for P2P connections on Bitcoin Mainnet
  • The 18333 port is used for P2P connections on Bitcoin Testnet
  • Port 8332 is used for the interface RPC of a Bitcoin node
  • Port 9050 is used by Tor's SOCKS5 proxy
  • Port 9150 is used by the Tor Browser
In other words, a port is like a dedicated door. When you visit a web site, your computer opens a source port and sends a request to the remote site's IP address, targeting port 443 if it's an encrypted connection. The remote server then returns the response via the same channel. On a router, this notion of port is important: It needs to know which device and which local port to redirect incoming data packets to.
But this ability to open ports also introduces a security risk. A port left open unnecessarily can be detected by network scanning tools and exploited by attackers if it is associated with a vulnerable or misconfigured service. It's like leaving a door open. But more on that later.
Understanding the basic operation of your home network is therefore very important. A good grasp of the basics of your modem, Wi-Fi, routers, local IP addresses and ports enables you to identify potential risks and act accordingly to strengthen your digital security.
In this chapter, we've covered the important concepts of home networking. In the next chapter, we'll look at how to secure your local Internet infrastructure in practice, and how to configure your Wi-Fi correctly to optimize your security.
Quiz
Quiz1/5
Which network port is used by default for P2P connections on the Bitcoin main network?