2809ICT Computer Networking Essentials – Comprehensive Revision

These question-and-answer flashcards cover the core concepts, definitions, commands, protocols and security practices discussed across all modules of 2809ICT Computer Networking Essentials, including networking fundamentals, switching, routing, subnetting, IPv6, LAN security, troubleshooting, and distributed ledger technologies.

What are the four basic characteristics of a reliable network architecture?

Fault tolerance, Scalability, Quality of Service (QoS), and Security.

In networking diagrams, what does a physical topology illustrate?

The physical location of devices and how they are interconnected.

What IOS command encrypts all plaintext passwords in the running-config?

service password-encryption

Which IOS mode is entered with the command "enable"?

Privileged EXEC mode (Switch# or Router#).

How is a /24 subnet mask written in dotted decimal notation?

255.255.255.0

Name the three elements that every communication system must include.

Source (sender), Destination (receiver), and Channel (transmission media).

Which three aspects are defined by a protocol?

Syntax, Semantics, and Timing.

Define Unicast, Multicast, and Broadcast message delivery.

Unicast: one-to-one; Multicast: one-to-many (selected); Broadcast: one-to-all (IPv4 only).

What two IP addressing parts exist in every IPv4 address?

Network portion and Host portion.

At Layer 2, which addresses does a frame carry when the source and destination are on the same LAN?

Source MAC = NIC of sender; Destination MAC = NIC of destination device on the same link.

What is the purpose of the Logical Link Control (LLC) sublayer?

To identify the Layer-3 protocol and allow multiple network layer protocols to share the same interface and media.

What Ethernet access method is used in half-duplex LANs?

CSMA/CD (Carrier Sense Multiple Access/Collision Detection).

Which wireless LAN access method attempts to avoid collisions rather than detect them?

CSMA/CA (Collision Avoidance).

How does a switch learn MAC addresses?

By examining the Source MAC address of incoming frames and storing it with the ingress port in the MAC address table.

What happens when a switch receives a frame with an unknown destination MAC?

The frame is flooded out all ports except the port it arrived on.

What is a broadcast domain and which device breaks it?

All devices that receive broadcast traffic on a LAN; a router (Layer-3 device) breaks broadcast domains.

List the three common VLAN types defined in class.

Data VLAN, Native VLAN, Management VLAN.

What command sets an access port to VLAN 20?

switchport access vlan 20

Which IEEE standard defines VLAN trunk tagging?

IEEE 802.1Q

What field in the 802.1Q header holds the VLAN ID?

The 12-bit VLAN ID (VID) field.

Name two Cisco-specific commands that disable DTP negotiations on a port.

switchport mode access (on access ports) and switchport nonegotiate (on trunk ports).

Why is STP required in a redundant Layer-2 topology?

To prevent Layer-2 loops that cause broadcast storms, MAC table instability, and link saturation.

List the four main steps STP uses to create a loop-free topology.

Elect Root Bridge, Elect Root Ports, Elect Designated Ports, Identify Alternate (blocked) Ports.

What is EtherChannel?

A technology that bundles multiple physical links into one logical link for redundancy and increased bandwidth without being blocked by STP.

Which three route source codes might appear in a Cisco IPv4 routing table?

C (Connected), S (Static), O (OSPF) – other codes include L, D, etc.

What type of route forwards traffic when there is no specific match in the routing table?

Default route.

Why must a Layer-2 switch be configured with a default gateway?

To allow remote management (e.g., SSH/Telnet) from outside its local network.

Explain "Router-on-a-Stick" inter-VLAN routing.

A single router interface configured as an 802.1Q trunk with multiple subinterfaces, each serving as a gateway for a different VLAN subnet.

What IPv4 header field prevents a packet from circulating indefinitely?

Time to Live (TTL).

Which IPv6 header field replaces the IPv4 TTL?

Hop Limit.

State the formula for maximum number of subnets created when borrowing bits.

2^b where b = number of borrowed bits.

State the formula for maximum usable hosts per subnet.

(2^r) – 2, where r = remaining host bits.

What is the first rule for shortening (compressing) an IPv6 address?

Omit leading zeros in any hextet.

What symbol (::) can replace one contiguous string of all-zero hextets in an IPv6 address?

A double colon (::) – but only once per address.

What is SLAAC?

Stateless Address Autoconfiguration – allows IPv6 hosts to self-configure a GUA using information from Router Advertisements.

Differentiate between Stateless and Stateful DHCPv6.

Stateless supplies supplemental info (DNS, domain) while host configures its own address via SLAAC; Stateful provides the IPv6 address, prefix, DNS, etc., similar to DHCPv4.

Describe the EUI-64 process used to create an IPv6 interface ID.

Insert FFFE in the middle of the 48-bit MAC and flip the 7th bit of the original MAC to 1.

Give two examples of Layer-2 security threats.

MAC address flooding (CAM table attack) and VLAN hopping.

Which feature helps mitigate DHCP starvation and spoofing attacks?

DHCP Snooping.

What Layer-2 security feature validates ARP messages before updating the ARP table?

Dynamic ARP Inspection (DAI).

List two best practices to mitigate VLAN hopping attacks.

Disable DTP on access ports and set the native VLAN to a non-default VLAN other than VLAN 1.

What are the six basic troubleshooting steps (Cisco methodology)?

1) Identify the problem, 2) Establish theory of probable causes, 3) Test the theory, 4) Establish plan of action & implement, 5) Verify solution & implement preventive measures, 6) Document findings.

Define distributed ledger technology (DLT) in one sentence.

A peer-to-peer, append-only system that provides a shared, tamper-evident record of transactions without a central authority.

Name three key properties blockchain aims to provide.

Immutability (tamper-evident records), Traceability (provenance), and Censorship resistance.

Which data structure is used in blockchains to summarise all transactions in a block?

Merkle tree (Merkle root stored in the block header).

What consensus mechanism does Bitcoin employ?

Proof of Work (PoW).

What prevents "double-spending" in Bitcoin?

Timestamped PoW chain and network consensus on the longest valid chain.

List four core mechanisms combined in blockchain systems.

Peer-to-Peer networking, Hash functions, Asymmetric encryption (digital signatures), and Consensus algorithms with incentives.

What is a smart contract?

Program code stored and executed on a blockchain, automatically enforcing agreed rules and transferring value/state when conditions are met.

Explain "UTXO" in Bitcoin.

Unspent Transaction Output – a chunk of bitcoin value that can be consumed as an input in a future transaction.

Why is the nonce field important in a Bitcoin block header?

Miners vary the nonce to produce a hash below the difficulty target, proving work for the block.

What happens if two miners simultaneously discover valid blocks (fork)?

Both blocks propagate, forming parallel chains; the longest chain (most cumulative PoW) eventually becomes the accepted chain.

Which three IPv6 dynamic addressing methods were covered?

1) SLAAC only, 2) SLAAC + Stateless DHCPv6, 3) Stateful DHCPv6 only.

In Cisco switches, what command converts an interface to a trunk manually?

switchport mode trunk

Name two advantages of using a layered network model.

Encourages vendor interoperability and isolates technology changes to individual layers.

Place these PDUs in order from Layer-7 down to Layer-1: Packet, Frame, Segment, Bits, Data.

Data, Segment, Packet, Frame, Bits.

What conversion operations between numbering systems should a student know for the exam?

Binary–Decimal, Binary–Hex, Decimal–Hex (and vice-versa), plus logical AND operation.

Give two properties that make fiber-optic cabling attractive for network backbones.

Longer distance transmission with low attenuation and immunity to EMI/RFI.

Which IEEE standards correspond to Wi-Fi, Bluetooth, and WiMAX?

Wi-Fi – 802.11; Bluetooth – 802.15; WiMAX – 802.16.

What does the switchport trunk native vlan command do?

Sets which VLAN’s frames are sent untagged on a trunk link.

Which STP standard is defined by IEEE?

IEEE 802.1D.

What Cisco proprietary protocol negotiates trunk links?

Dynamic Trunking Protocol (DTP).

Why is NAT considered a limitation of IPv4?

It breaks true end-to-end connectivity, adds complexity, latency, and complicates troubleshooting.

What is the main goal of Variable Length Subnet Masking (VLSM)?

To allocate IP address space efficiently by creating subnets of different sizes based on host requirements.

Describe "Fault Tolerance" in network architecture.

The ability of a network to continue operating properly even when a component fails.

What command shows the routing table on a Cisco router?

show ip route

Which field in the IPv4 header is used for QoS marking?

Differentiated Services (DS) field.

What Cisco attack mitigation technique prevents IP/MAC spoofing on a port?

IP Source Guard (IPSG).

Which security practice secures device management traffic within a dedicated VLAN?

Using a management VLAN and restricting it with ACLs or out-of-band management.

What process is used to convert data down the OSI stack before transmission?

Encapsulation.

Define "Scalability" in the context of network design.

The capability to grow and adapt to increased demand without reducing performance.

What is meant by "Quality of Service"?

Techniques to manage network resources and guarantee performance (delay, bandwidth, jitter) for critical traffic.

Why are broadcast frames forwarded by switches but typically blocked by routers?

Switches operate at Layer-2 and forward broadcasts within a LAN; routers operate at Layer-3 and do not forward Layer-2 broadcasts, thereby containing broadcast domains.

What advantage does EtherChannel provide during link failure?

The logical bundle stays up, so traffic automatically redistributes across remaining active links without STP reconvergence.

Which subnet ID field in IPv6 addressing allows easy subnetting within an organisation?

The 16-bit subnet ID field between the Global Routing Prefix and Interface ID.

What is "Anycast" in IPv6?

An address assigned to multiple devices where traffic is routed to the nearest (topologically) device holding that address.

Give one example where an out-of-band management network is useful.

Managing switches/routers even when the production network is down or under attack.

Name two common consensus algorithms besides Proof of Work.

Proof of Stake and Practical Byzantine Fault Tolerance (PBFT).

What property do hash functions provide that is crucial for blockchain integrity?

Collision resistance and the ability to verify data integrity with fixed-length outputs.

Why is formal verification important in DLT systems?

To mathematically prove protocol correctness and reduce vulnerabilities in implementation.

What Cisco feature enforces a maximum number of MAC addresses on a switch port?

Port Security (with static, sticky, or dynamic MAC limits).