Introduction to LAN
Learn about some of the technologies and designs that power private networks
LAN Topologies
LAN (Local Area Network). In reference to networking, when we refer to the term "topology", we are actually referring to the design or look of the network at hand.
Star Topology
The main premise of a star topology is that devices are individually connected via a central networking device such as a switch or hub. This topology is the most commonly found today because of its reliability and scalability - despite the cost.
Because more cabling and the purchase of dedicated networking equipment is required for this topology, it is more expensive than any of the other topologies. However, despite the added cost, this does provide some significant advantages. For example, this topology is much more scalable in nature, which means that it is very easy to add more devices as the demand for the network increases.
Unfortunately, the more the network scales, the more maintenance is required to keep the network functional. This increased dependence on maintenance can also make troubleshooting faults much harder. Furthermore, the star topology is still prone to failure - albeit reduced. For example, if the centralised hardware that connects devices fails, these devices will no longer be able to send or receive data. Thankfully, these centralised hardware devices are often robust.
Bus Topology
This type of connection relies upon a single connection which is known as a backbone cable. This type of topology is similar to the leaf off of a tree in the sense that devices (leaves) stem from where the branches are on this cable.
Because all data destined for each device travels along the same cable, it is very quickly prone to becoming slow and bottlenecked if devices within the topology are simultaneously requesting data. This bottleneck also results in very difficult troubleshooting because it quickly becomes difficult to identify which device is experiencing issues with data all travelling along the same route.
Bus topologies are one of the easier and more cost-efficient topologies to set up because of their expenses, such as cabling or dedicated networking equipment used to connect these devices. Another disadvantage of the bus topology is that there is little redundancy in place in case of failures. If the backbone cable were to break, devices can no longer receive or transmit data along the bus.
Ring Topology
Devices such as computers are connected directly to each other to form a loop, meaning that there is little cabling required and less dependence on dedicated hardware such as within a star topology.
A ring topology works by sending data across the loop until it reaches the destined device, using other devices along the loop to forward the data. Interestingly, a device will only send received data from another device in this topology if it does not have any to send itself. If the device happens to have data to send, it will send its own data first before sending data from another device.
It is fairly easy to troubleshoot any faults that arise. However, this is a double-edged sword because it isn't an efficient way of data travelling across a network, as it may have to visit many multiple devices first before reaching the intended device. They are less prone to bottlenecks, such as within a bus topology, as large amounts of traffic are not travelling across the network at any one time. The design of this topology does, however, mean that a fault such as cut cable, or broken device will result in the entire networking breaking
Switches
Switches are dedicated devices within a network that are designed to aggregate multiple other devices such as computers using ethernet. These various devices plug into a switch's port. Switches are usually found in larger networks where there are many devices to connect to the network. Switches can connect a large number of devices by having ports of 4, 8, 16, 24, 32, and 64 for devices to plug into.
Switches are much more efficient than their lesser counterpart (hubs/repeaters). Switches keep track of what device is connected to which port. This way, when they receive a packet, instead of repeating that packet to every port like a hub would do, it just sends it to the intended target, thus reducing network traffic.
Both Switches and Routers can be connected to one another. The ability to do this increases the redundancy (the reliability) of a network by adding multiple paths for data to take. If one path goes down, another can be used. Whilst this may reduce the overall performance of a network because packets have to take longer to travel, there is no downtime.
Routers
It's a router's job to connect networks and pass data between them. It does this by using routing (hence the name router!).
Routing is the label given to the process of data travelling across networks. Routing involves creating a path between networks so that this data can be successfully delivered. Routing is useful when devices are connected by many paths.
Subnetting
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.
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 3 ways:
Identify the network address
Identify the host address
Identify the default gateway
Subnetting is beneficial for efficiency, security and full control. It allows you to separate two use cases from each other whilst having the benefits of a connection to larger networks such as the Internet.
Example: A café has two networks:
One for employees, cash registers, and other devices for the facility
One for the general public to use as a hotspot
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.
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.
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).
ARP Protocol
The protocol is the technology that is responsible for allowing devices to identify themselves on a network. It 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 the ARP protocol to find the MAC address (and therefore the physical identifier) of a device for communication.
Each device within a network has a ledger to store information on, which is called a cache. In the context of the ARP protocol, this cache stores the identifiers of other devices on the network. In order to map these two identifiers together (IP address and MAC address), the ARP protocol sends two types of messages:
ARP Request
ARP Reply
When an ARP request is sent, a message is broadcasted to every other device found on a network by the device, asking whether or not the device's MAC address matches the requested IP address. If the device does have the requested IP address, an ARP reply is returned to the initial device to acknowledge this. The initial device will now remember this and store it within its cache (an ARP entry).
MAC addresses are used as the physical identifiers and IP addresses are used as the logical identifiers for a device on a network.
DHCP Protocol
IP addresses can be assigned either manually, by entering them physically into a device, or automatically and most commonly by using a 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).
Answers
Task 1
What does LAN stand for? Local Area Network
What is the verb given to the job that Routers perform? Routing
What device is used to centrally connect multiple devices on the local network and transmit data to the correct location? Switch
What topology is cost-efficient to set up? Bus Topology
What topology is expensive to set up and maintain? Star Topology
Complete the interactive lab attached to this task. What is the flag given at the end? THM{TOPOLOGY_FLAWS}
Task 2
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? 0–225
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
Task 3
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
Task 4
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
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