Table of Content
Introduction LAN Toplogies
Local Area Network (LAN)
Star Topology
Advantages:
- Reliability: Individual device failures don’t bring down the entire network.
- Scalability: Easy to add or remove devices.
- Easy Troubleshooting: Faults are generally easier to isolate.
Disadvantages:
- Cost: Requires more cabling and a central switch/hub, increasing initial investment.
- Single Point of Failure: Failure of the central device disables the entire network.
- Increased Maintenance: Larger networks require more maintenance.
Bus Topology
Advantages:
- Cost-Effective: Relatively inexpensive to implement due to minimal cabling and equipment.
- Easy to Install: Simple setup and configuration.
Disadvantages:
- Performance Bottlenecks: A single cable becomes a bottleneck with increased network traffic.
- Difficult Troubleshooting: Identifying the source of problems can be challenging.
- Single Point of Failure: A break in the backbone cable disables the entire network.
Ring Topology
Advantages:
- Equitable Access: Data transmission is managed in a fair manner.
- Easy Troubleshooting (in one direction): Fault identification can be relatively straightforward due to unidirectional data flow.
- Less prone to bottlenecks (compared to Bus): Data flow is managed to avoid congestion as seen in bus topology.
Disadvantages:
- Inefficient Data Transfer: Data may have to travel through multiple devices before reaching its destination.
- Single Point of Failure: A single cable or device failure disrupts the entire network.
- Difficult to add or remove devices: Disrupting the ring to add/remove a device is required.
Network Devices: Switches and Routers
Switches
- Function: Connect multiple devices on a LAN, forwarding data only to the intended recipient. More efficient than hubs.
- Benefits: Reduced network congestion, improved performance.
Routers
- Function: Connect different networks, determining the best path for data transmission (routing).
- Benefits: Enables communication between different networks, increases network redundancy when used with multiple paths.
Practical Exercise (Instructions)
The practical exercise involves identifying vulnerabilities in each topology and “breaking” them (simulated) to retrieve a flag. Specific instructions for this practical would be provided separately, as they depend on the interactive nature of the exercise. However, the vulnerabilities mentioned above are key considerations when attempting to complete the exercise (e.g., focusing on single points of failure for bus and ring, or the central device for star).
A Primer on Subnetting
As we’ve previously discussed throughout the module so far, Networks can be found in all shapes and sizes - ranging from small to large. Subnetting is the term given to splitting up a network into smaller, miniature networks within itself. Think of it as slicing up a cake for your friends. There’s only a certain amount of cake to go around, but everybody wants a piece. Subnetting is you deciding who gets what slice & reserving such a slice of this metaphorical cake.
Take a business, for example; You will have different departments such as:
- Accounting
- Finance
- Human Resources
Whilst you know where to send information in real life to the correct department, networks need to know as well. Network administrators use subnetting to categorise and assign specific parts of a network to reflect this.
Subnetting is achieved by splitting up the number of hosts that can fit within the network, represented by a number called a subnet mask. Let’s refer back to our diagram from the first room in this module:
As we can recall, an IP address is made up of four sections called octets. The same goes for a subnet mask which is also represented as a number of four bytes (32 bits), ranging from 0 to 255 (0-255).
Subnets use IP addresses in three different ways:
- Identify the network address
- Identify the host address
- Identify the default gateway
Let’s split these three up to understand their purposes into the table below:
| Type | Purpose | Explanation | Example |
|---|---|---|---|
| Network Address | This address identifies the start of the actual network and is used to identify a network’s existence. | For example, a device with the IP address of 192.168.1.100 will be on the network identified by 192.168.1.0 | 192.168.1.0 |
| Host Address | An IP address here is used to identify a device on the subnet | For example, a device will have the network address of 192.168.1.1 | 192.168.1.100 |
| Default Gateway | The default gateway address is a special address assigned to a device on the network that is capable of sending information to another network | Any data that needs to go to a device that isn’t on the same network (i.e. isn’t on 192.168.1.0) will be sent to this device. These devices can use any host address but usually use either the first or last host address in a network (.1 or .254) | 192.168.1.254 |
Now, in small networks such as at home, you will be on one subnet as there is an unlikely chance that you need more than 254 devices connected at one time.
However, places such as businesses and offices will have much more of these devices (PCs, printers, cameras and sensors), where subnetting takes place.
Subnetting provides a range of benefits, including:
- Efficiency
- Security
- Full control
We’ll come on to explore exactly how subnetting provides these benefits at a later date; however, for now, all we need to understand is the security element to it. Let’s take the typical café on the street. This cafe will have two networks:
- One for employees, cash registers, and other devices for the facility
- One for the general public to use as a hotspot
Subnetting allows you to separate these two use cases from each other whilst having the benefits of a connection to larger networks such as the Internet.
Questions
- What is the technical term for dividing a network up into smaller pieces?
- Subnetting
- How many bits are in a subnet mask?
- 32
- What is the range of a section (octet) of a subnet mask?
- 0-255
- What address is used to identify the start of a network?
- Network Address
- What address is used to identify devices within a network?
- Host Address
- What is the name used to identify the device responsible for sending data to another network?
- Default Gateway
ARP
Recalling from our previous tasks that devices can have two identifiers: A MAC address and an IP address, the Address Resolution Protocol or ARP for short, is the technology that is responsible for allowing devices to identify themselves on a network.
Simply, ARP allows a device to associate its MAC address with an IP address on the network. Each device on a network will keep a log of the MAC addresses associated with other devices.
When devices wish to communicate with another, they will send a broadcast to the entire network searching for the specific device. Devices can use ARP to find the MAC address (and therefore the physical identifier) of a device for communication.
How does ARP Work?
Each device within a network has a ledger to store information on, which is called a cache. In the context of ARP, this cache stores the identifiers of other devices on the network.
In order to map these two identifiers together (IP address and MAC address), ARP sends two types of messages:
- ARP Request
- ARP Reply
When an ARP request is sent, a message is broadcasted on the network to other devices asking, “What is the mac address that owns this IP address?” When the other devices receive that message, they will only respond if they own that IP address and will send an ARP reply with its MAC address. The requesting device can now remember this mapping and store it in its ARP cache for future use.
This process is illustrated in the diagram below:
Questions
- What does ARP stand for?
- Address Resolution Protocol
- What category of ARP Packet asks a device whether or not it has a specific IP address?
- Request
- What address is used as a physical identifier for a device on a network?
- MAC Address
- What address is used as a logical identifier for a device on a network?
- IP Address
DHCP
IP addresses can be assigned either manually, by entering them physically into a device, or automatically and most commonly by using a DHCP (Dynamic Host Configuration Protocol) server. When a device connects to a network, if it has not already been manually assigned an IP address, it sends out a request (DHCP Discover) to see if any DHCP servers are on the network. The DHCP server then replies back with an IP address the device could use (DHCP Offer). The device then sends a reply confirming it wants the offered IP Address (DHCP Request), and then lastly, the DHCP server sends a reply acknowledging this has been completed, and the device can start using the IP Address (DHCP ACK).
Questions
- What type of DHCP packet is used by a device to retrieve an IP address?
- DHCP Discover
- What type of DHCP packet does a device send once it has been offered an IP address by the DHCP server?
- DHCP Request
- Finally, what is the last DHCP packet that is sent to a device from a DHCP server?
- DHCP ACK
Next
Continue your learning by joining the “OSI Model” room.