Defining Common Network Terms
Originally Posted: April 21st, 2020
Last Edited: August 29th, 2022
Defining Common Network Terms
Table of Contents
A lot of people misuse common networking terms. To avoid confusion, let’s define them:
Core Network Terms
- A Network is a collection of devices that are connected to enable communication.
- A Node is a single device in a network.
- A Client is a user of a network.
- Wi-Fi is a type of radio wave that carries data.
- Ethernet is a common cabling standard for wired networks.
Core Network Devices
A Modem modulates and demodulates a signal
In English, that means it takes the signal from your Internet provider and converts it into a signal you can use with other networking equipment. Internet service and required equipment varies. In the US, most people who have internet from their telephone or cable provider require a modem to get access to the Internet.
The modem takes the signal delivered over telephone or coaxial cable and converts it into a usable Internet signal, usually in the form of Ethernet. Fiber providers like Verizon FiOS usually refer to this as an ONT — Optical Network Terminal. The ONT takes the fiber from your provider and converts it to Ethernet.
A Router takes packets from one network and sends them to another
In a common analogy, packets are letters, and the router is the post office. The router is what decides what goes where. Routers are usually all about getting packets delivered, but they can also deny delivery. Denying delivery of packets is usually the role of a firewall.
In the OSI model, routers speak on layer 3, working with IP addresses and IP subnets. To get from your internal, private, 192.168.x.x network and onto the public Internet, you need routing and some NAT (network address translation).
A Firewall is the traffic cop on the network
Firewalls are focused on security. Usually firewalls are placed at the edge of your network, and monitor what traffic is coming in and what traffic is leaving. They can also police internally, preventing guests from accessing your internal resources, or selectively allowing access to important devices. Firewalls often perform other security functions, such as acting as a VPN server to allow secure remote access to a network.
A Switch multiplies ports
Switches can be a lot more, but for our purposes, switches are port multipliers. They let you connect more devices to your wired network. Switches are typically layer 2 devices, speaking in MAC addresses and VLANs, not IP subnets or IP Addresses like routers.
Managed switches let you separate your network on layer 2, separating devices into different virtual networks (VLANs). To communicate between VLANs, you need a device that can speak in layer 3 — a router, firewall or L3 switch.
An Access Point creates a wireless signal
Wireless access points typically take in a wired Ethernet connection, and convert it into radio waves, known as Wi-Fi. Most access points get their power and path to the network over Ethernet, and then provide client devices with connectivity over a wireless link. They essentially act as a layer 2 bridge, converting wired to wireless.
A Mesh network is a collection of access points that provide a single wireless network
Access points can utilize wireless backhaul, allowing it to use radio waves for it’s communication to both the network and the client. This is referred to as a wireless mesh — multiple access points communicating wirelessly to coordinate the function of the wireless network.
All-In-One devices can do multiple, or all of these things
Most people refer to their all-in-one device as their “router”, but that doesn’t reflect everything that device actually does. Most consumer “routers” are actually an all-in-one router, switch, and Wi-Fi access point. Some also add in the required modem or signal conversion into that same box. In the US, an example of who does this is Comcast. If you have a device that takes in Coaxial cable, and provides Internet, Ethernet ports, and wireless, that’s doing more than just routing.
A Wireless Repeater rebroadcasts a wireless signal
Wireless repeaters take an existing wireless network and rebroadcasts it to extend its range. They are essentially access points that use wireless backhaul to your main wireless router or access point instead of Ethernet. This is similar to how nodes in a mesh network operate. The difference is that wireless repeaters are not part of the same control plane, and are not managed by the same system as the base access point. This leads to poor performance and unreliability. They allow you to extend the reach of your network, but this is one of the worst ways to do that.
A Powerline Adapter uses in-wall power cabling as network cabling
Powerline adapters uses in-wall power cabling to transmit data signals. Typically you connect one end to a switch/router, and the other adapter when you need an Ethernet connection for a client device. They often advertise high speeds, but in practice you will only ever get close to that speed if you plug them in adjacent sockets. Real-life efficiency around 30% (300 Mbps from a 1 Gbps link) or lower is common, and speeds aren’t always consistent. Powerline adapters typically need to share a breaker, and can produce a lot of radio noise. Only use a powerline adapter if you are unable to run Ethernet or coax cabling instead.
A MoCA Adapter uses Coax cabling as network cabling
These adapters let you run Ethernet over existing coax cabling in your house. The performance of these devices are usually very good and are superior to powerline adapters. If possible, dedicated Ethernet runs are a better solution, but if you already have coax cabling installed and installing Ethernet is a hassle, this are a perfectly viable alternative.
A Wireless Bridge creates a point-to-point link
Wireless bridges create a point-to-point link, usually over a larger distance, like between two buildings. Ethernet is limited to 100 meters, and fiber can be expensive, or require expensive equipment to install. In situations like that, a wireless bridge might be a better solution. You can think of a wireless bridge as essentially acting as a wireless cable. At the remote end, you get an Ethernet jack that you can hook up to a switch or access point to extend your network to that location.
Wireless bridges are a very inexpensive way to extend your network to a specific location that can be several kilometers away. They provide pretty good performance, but require line of sight. Obstacles between the nodes will dramatically reduce signal quality and strength.
Networks can be broken down into 7 layers, with different devices and functions happening on those different layers. Let’s look at the bottom three layers first, and work our way up.
Layer 1 = Physical
Cables, radio waves, etc.
Binary 1’s and 0’s, electrical pulses, and radio waves operate at this level, where they exchange raw Bits of binary information.
Layer 2 = Data Link
Communicating on a network using Physical Addresses like a MAC address.
At layer 2 you care about the hardware address of the device you’re directly connected to.
Switches, VLANS, Wi-Fi access points, and MAC addresses operate at this level, where they exchange groups of bits in the form of Frames.
Layer 3 = Network
Communicating on a network using Logical Addresses like an IP address.
At layer 3 you define IP addresses for devices, and can route between many connected networks.
Routers, subnets, and IP addresses operate at this level, where they exchange groups of bits in the form of Packets.
Routers separate Layer 3 networks, also known as subnets
A subnet is defined by it’s IP addressing. A common subnet is 192.168.1.0/24.
Switches operate at Layer 2, forming LANs and VLANs
A LAN is a local area network. LANs are usually defined by their physical connections.
A VLAN is a virtual LAN. VLANs are defined by their logical VLAN ID number, from 1 to 4096.
A Broadcast Domain is a logical division of a network
In a broadcast domain, all nodes can reach each other by broadcasting.
Layer 2 broadcast domain = LAN or VLAN
Layer 3 broadcast domain = Subnet
A LAN is a single Layer 2 network, with a single Layer 2 broadcast domain
Picture a single Ethernet switch. You plug a bunch of devices in, and they can all communicate directly to each other. You can plug in more switches and more devices, but the scale of a LAN is usually limited to a single building or a few hundred devices at most.
Your home network is usually a single LAN. If you wanted to communicate with your neighbor or someone on the Internet, you’d need to leave your LAN using a layer 3 device like a router.
A VLAN is a virtual Layer 2 network, or a virtual LAN
A VLAN can be thought of as a virtual switch. VLANs allow you to mix and match which Layer 2 network individual ports and devices use. Instead of separating these devices onto different physical switches, we can do the same thing by defining multiple VLANs on a single physical switch.
Just like with LANs, devices on the same VLAN can talk to each other directly. Devices on different VLANs need to go through a router to communicate.
A Subnet is a single Layer 3 network, with a single Layer 3 broadcast domain
For most people, their home network is a single subnet. It’s usually represented by an IP address like 192.168.1.0/24.
All devices using an IP address beginning with 192.168.1 belong to the 192.168.1.0/24 subnet.
192.168.1.1 is your router, 192.168.1.2 is your phone, 192.168.1.3 is your laptop, etc.
All devices in your home can broadcast to each other at layer 3. Your router is the border between your internal network and the Internet, stopping broadcast traffic in both directions.
Subnets and VLANs can be tied together to separate devices at Layer 2 and 3
As an example, let’s look at two VLANs, 10 and 20. We can define VLAN 10 as 192.168.10.0/24, and VLAN 20 as 192.168.20.0/24.
Devices in the same VLAN or subnet can communicate directly with each other, as if they were plugged into the same physical switch.
For VLAN 10 devices to reach VLAN 20 devices, they need a router to relay their messages between networks.
Separation at Layer 3 allows you to define security policies with your router or firewall.
You can prevent communication between the networks, narrowly define which types are allowed, or restrict the source or destination of traffic.
Subnets allow you to define which devices should be grouped together. VLANs allow you to selectively apply that subnet to individual devices, switches, and ports. This is why VLANs are usually tied to subnets, and why it’s necessary to separate networks on both layers.
Security at layer 2 is more limited, and MAC address-based security is easily spoofed or bypassed. Layer 3 and up are where the most valuable security methods exist.
The OSI Model standardizes and classifies the functions of a network
The OSI model.
Layer 1 = physical layer
Function: Transmission and reception of raw bit streams over a physical medium. Converting digital bits into electrical, radio, or optical signals.
Operates at Layer 1: Hubs, cables, bits, volts, radio waves. Implementation of wired and wireless standards for Ethernet, Wi-Fi, Bluetooth, coax, fiber, etc.
Protocol Data Unit (PDU): Symbols
Later 2 = Data Link
Function: Reliable transmission of data frames between two nodes connected by a physical layer.
Operates at Layer 2: Switches, bridges, APs, MAC addresses, VLANs, Ethernet, Wi-Fi, and ZigBee.
PDU: Frames
Layer 3 = Network
Function: Structuring and managing a multi-node network, including addressing, routing and traffic control.
Operates at Layer 3: Routers, firewalls, IP Addresses and subnets.
PDU: Packets
Layer 4 = Transport
Function: Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing.
Operates at Layer 4: TCP, UDP, segmentation, flow control, windowing, acknowledgment.
PDU: Segments
Layer 5 = Session
Function: Managing communication sessions, i.e., continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes.
Operates at Layer 5: Session establishment, maintenance and termination. Logical ports, remote procedure calls, L2TP, PPTP, RTC, SCP, etc.
PDU: Data
Layer 6 = Presentation
Function: Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption.
Operates at Layer 6: File formats like gif, jpeg, ASCII. Encryption and decryption, encoding of data into a usable format.
PDU: Data
Layer 7 = Application
Function: High-level APIs, including resource sharing, remote file access. This layer interacts with software applications that implement a communicating component.
Operates at Layer 7: HTML, DNS, SMTP, FTP, etc. The protocols that are used by user applications like web browsers or text editors.
PDU: Data