Network Layer

• IP (Internet Protocol): provides addressing and routing of packets ("best-effort" delivery, no reliability guarantee — reliability is TCP's job, not IP's)
• IPv4: 32-bit addresses (~4.3 billion addresses); Address classes: Class A (large networks), B (medium), C (small), D (multicast), E (experimental)
• Special IP addresses: loopback (127.0.0.1), private address ranges (e.g., 192.168.x.x), broadcast address
• ARP (Address Resolution Protocol): resolves an IP address to its corresponding MAC (hardware) address on a local network
• IPv6: 128-bit addresses, designed to solve IPv4 address exhaustion
• ICMP: used for network diagnostics/error reporting (e.g., ping, traceroute)
• NAT (Network Address Translation): allows multiple devices on a private network to share a single public IP address
• Routing protocols: distance-vector vs link-state algorithms
• WAN technologies: X.25, Frame Relay, ATM (fixed-size cells), MPLS (speeds up routing using labels instead of full address lookups at each hop)

MCQ Trap

• ARP resolves IP→MAC; DNS resolves domain name→IP — both are "resolution" protocols but at different levels, easily confused.