Application Layer (User-level services)
FTP (Port 21) → File transfer
SSH (Port 22) → Secure remote login
TELNET (Port 23) → Remote login (insecure)
SMTP (Port 25) → Send emails
DNS (Port 53) → Domain to IP conversion
HTTP (Port 80) → Web browsing (insecure)
POP3 (Port 110) → Receive emails
HTTPS (Port 443) → Secure web browsing
MTP → Media file transfer
QOTD → Quote of the day (rarely used)
Transport Layer (End-to-end communication)
TCP → Reliable data transfer (acknowledged, ordered)
UDP → Fast, lightweight transfer (no guarantee)
Network & Data Link Layer (Addressing & delivery)
ARP → Finds MAC address from IP
RARP → Finds IP from MAC address
PPP → Point-to-point connection between two devices
