DNS — Hierarchy, Records & TTL

Name → IP via a hierarchical distributed database. Client asks recursively; the resolver iterates: root → TLD → authoritative, then caches the answer per TTL. UDP/53 for queries, TCP/53 for zone transfers, DNSSEC and truncated answers. Record Purpose A / AAAA IPv4 / IPv6 CNAME alias MX mail exchangers NS authoritative servers PTR reverse (IP→name) […]

NTP — Strata, Peers & Authentication

UDP 123, hierarchical strata (0 = reference clock, 1 = attached server, … max 15). Clients measure offset + delay and slew the clock (small steps) rather than jumping it. NTP delivers UTC — local display is clock timezone. Mode Direction Use ntp server unidirectional normal client/server hierarchy ntp peer bidirectional two cores syncing each […]

DHCP — DORA, Relay & Options

UDP 67 (server) / 68 (client). IPv4 clients start with a broadcast DORA: Discover → Offer → Request → Ack. Renew unicast at 50 % of the lease, broadcast at 87.5 %, full DORA when expired. Relay — the cross-subnet piece Broadcasts die at routers, so the first L3 hop relays: ip helper-address 192.168.100.10 converts […]

ARP — Requests, Caches & Gratuitous

ARP maps IPv4 → MAC inside one broadcast domain: request is a broadcast “who has 192.168.1.20?”, the reply is a unicast. Stateless, unauthenticated, and the first packet pays the cost — everything else hits the ARP cache (~4 h aging on Cisco). The exam-grade details Routers ARP for the next-hop IP, never the final destination. […]