Computer Networks

The 'last mile' refers to the final leg of the telecommunications network from the provider's central office to the customer premises. This includes the local loop (the physical copper/fiber lines). The demarcation point is where provider responsibility ends, typically at the customer's building entrance.

A supernet is a summarized route that combines multiple contiguous classful networks into a single larger network block. This reduces routing table size. It is not a default route (0.0.0.0/0), does not mix public/private addresses, and represents contiguous networks.

OSPF routers must have matching hello and dead intervals to form an adjacency. Different intervals prevent routers from recognizing each other as valid neighbors. Process IDs are locally significant and can differ, different router IDs are expected and necessary, and multiaccess networks work fine with proper DR/BDR election.

A rollover cable (also called a console cable) is used to connect a PC terminal to a router's console port. The cable pins are reversed at one end (pin 1 connects to pin 8, pin 2 to pin 7, etc.), which is required for the console connection. Straight-through cables connect different devices, while crossover cables connect similar devices.

An Autonomous System (AS) is defined as a collection of networks under a single administrative entity that presents a common routing policy to the internet. This is the standard definition used in BGP and inter-domain routing. AS numbers identify these systems globally.

Telnet transmits all data (including usernames and passwords) in clear text over the network, making it vulnerable to eavesdropping and packet sniffing attacks. SSH encrypts the entire session, providing confidentiality and integrity. Telnet is widely available (option A is wrong), uses minimal bandwidth (option C is wrong), and does support authentication (option D is wrong - you authenticate with username/password, just not securely). The lack of encryption is Telnet's fundamental security weakness.

Data link layer protocols like PPP (Point-to-Point Protocol) and HDLC (High-Level Data Link Control) define how data is encapsulated into frames for transmission across WAN links. They handle framing, error detection, and authentication. Option A is incorrect because ISDN is circuit-switched but ATM is a cell-switched technology. Option C describes circuit-switching, not packet-switching. Option D is wrong because Frame Relay switches are service provider equipment, not customer premises equipment maintained by local administrators.

Circuit-switched WAN implementations require establishing a dedicated physical circuit through the network before any data transfer can begin. This call setup time creates a delay before communication can start, which is inefficient for bursty data traffic compared to packet-switched networks. The circuit must be fully established end-to-end first, then data flows, then the circuit is torn down - this setup/teardown overhead is the key drawback. Voice traffic is actually well-suited to circuit switching.

If Computer A can communicate with Computer B (likely on the same switch) but not with C or D (on the other switch), the trunk link between switches isn't carrying VLAN 50 traffic. By default, trunks carry all VLANs unless explicitly restricted. If VLAN 50 was previously removed from the allowed list or the trunk wasn't properly configured, inter-switch communication for that VLAN fails. Native VLAN mismatch (A) would cause different errors. A non-trunked link (B) wouldn't allow any VLAN traffic to pass.

ESS (Extended Service Set) is a wireless topology where multiple access points are connected to the same wired network and share the same SSID, enabling seamless roaming. IBSS (Independent Basic Service Set), also called ad hoc mode, is a peer-to-peer network between wireless devices without an access point. Ad hoc (C) is the same as IBSS. Only ESS (A) supports roaming between access points because stations can move between APs while maintaining connectivity to the same network.

When network services fail, in-band management access typically uses the Ethernet port. The console port (C) requires physical access and a console cable, which isn't always available. AUX (A) is a modem port for remote out-of-band access. Telnet (D) and SSH (E) are application-layer protocols that require network services to be functioning. If network services are down, you can't Telnet or SSH. The Ethernet port (B) is the correct answer because it provides in-band management access and doesn't depend on the failed network services - you can connect directly to the router's Ethernet interface for management.

WiMAX (Worldwide Interoperability for Microwave Access) is defined in IEEE 802.16 and provides broadband wireless access with speeds and quality comparable to DSL and cable. It offers last-mile broadband connectivity and can cover large geographic areas, making it an alternative to wired broadband connections.