Application Layer Protocols

DNS (Domain Name System)

DNS translates domain names into IP addresses using a distributed hierarchy: root servers → TLD servers (.com, .in) → authoritative servers. Resolution can be recursive or iterative.

Terminal

nslookup example.com

Name: example.com Address: 203.0.113.10 (example address)

Record types: A (IPv4), AAAA (IPv6), CNAME (alias), MX (mail server), NS (name server), TXT.

HTTP

HTTP is a stateless request–response protocol. Methods: GET, POST, PUT, PATCH, DELETE, HEAD.

Status code	Meaning
1xx	Informational
2xx	Success (200 OK, 201 Created)
3xx	Redirection (301, 304)
4xx	Client error (400, 401, 403, 404)
5xx	Server error (500, 503)

HTTP/1.0 uses non-persistent connections; HTTP/1.1 makes them persistent; HTTP/2 adds multiplexing; HTTP/3 runs over QUIC (UDP). HTTPS is HTTP over TLS.

Email Protocols

SMTP: sends and relays mail (push).
POP3: downloads mail, usually deleting it from the server.
IMAP: keeps mail on the server and syncs across devices.

Other Protocols

DHCP assigns IP addresses automatically (DORA: Discover, Offer, Request, Acknowledge). FTP uses separate control (21) and data (20) connections.

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