Lesson 44: Core Network Services Review

DNS (Domain Name System - UDP/53)

DNS is the internet's phonebook. It translates human-readable domain names (e.g., cisco.com) into numerical IP addresses (e.g., 192.0.2.1).

• The Query Process: A client asks its local DNS server, which recursively searches the root, TLD (Top-Level Domain), and authoritative servers until the mapping is found.

ARP (Address Resolution Protocol)

ARP is a Layer 2 protocol that maps an IPv4 address to a corresponding MAC address within the same local subnet.

• Process: If Host A needs to send a packet to Host B (same subnet) but only knows B's IP, A broadcasts an ARP request: "Who has 192.168.1.10? Tell 192.168.1.5."
• Host B responds with an ARP reply containing its MAC address.

ICMP (Internet Control Message Protocol - Layer 3)

ICMP is used for error reporting and network diagnostics, but not for carrying user data.

• Key Uses: ping (Echo Request/Reply) and traceroute/tracert (used for path discovery and error messages related to unreachable destinations or TTL expiration).