Learn

Protocols and Ports

The rules applications use to talk, and the port numbers that help you recognize those conversations.

Match common services with the ports they actually use
Separate secure protocols from legacy cleartext ones
Notice when a service uses more than one port or transport

A resolver walks the hierarchy

People type names. The resolver follows referrals from the root to the name's authority, then caches the answer for the stated time to live. This system publishes mappings. It does not lease a device its address.

Quiz this category Flashcards Build a name lookup

Transport

TCP
A reliable, ordered transport protocol that retransmits lost data.
UDP
A connectionless transport protocol that does not retransmit lost datagrams.

Resolution

ARP
Resolves a local 32-bit address to a hardware address.

Name services

DNS
The directory that translates names into addresses and other records.

Address configuration

DHCP
Leases addresses and basic network settings to clients automatically.

Web

HTTP
The web transfer protocol, unencrypted unless it is wrapped in encryption.
HTTPS
Web traffic protected by transport encryption and a certificate.
QUIC
An encrypted transport over UDP that carries the newest major web version.

Remote access

RDP
A graphical remote-desktop protocol, commonly on port 3389.
SSH
An encrypted protocol for remote login and command execution.
Telnet
A legacy remote terminal that sends keystrokes in the clear.

File transfer

FTP
A file-transfer protocol with a separate control connection on port 21.
FTPS
Classic file transfer wrapped in transport encryption.
SFTP
File transfer that runs inside an encrypted secure shell.
TFTP
A minimal unauthenticated file transfer, used on trusted local networks.

Email

IMAP
A mailbox protocol that keeps messages and folders on the server.
POP3
A mailbox protocol that downloads messages to the client.
SMTP
The protocol that transfers email between mail servers.

File sharing

SMB
A file- and printer-sharing protocol common on Windows networks.

Management

SNMP
A protocol for monitoring network devices and receiving their alerts.
Syslog
A standard for sending log messages to a collector.

Infrastructure

NTP
A protocol that synchronizes computer clocks over the network.

Diagnostics

ICMP
The diagnostic and error protocol used by connectivity tests. It has no ports.

Encryption

IPsec
A network-layer suite that authenticates and encrypts packets.
SSL
The obsolete predecessor of modern transport encryption.
TLS
A protocol that encrypts a session and checks the server's certificate.

Routing

BGP
The routing protocol that exchanges reachability between large networks.
OSPF
An interior link-state routing protocol that builds a map of an area.

Ports

Dynamic Ports
Short-lived source ports a client chooses for one conversation.
Registered Ports
Ports 1024 through 49151, available for registered applications.
Socket
An endpoint of a conversation, identified by address, port, and protocol.
Well-Known Ports
Ports 0 through 1023, reserved for standard services.

Security

Secure and Insecure Protocols
The contrast between encrypted protocols and cleartext predecessors.

Identity

Kerberos
A ticket-based authentication system built around a trusted key-distribution center.
RADIUS
A central authentication and accounting service for network access.
TACACS+
A device-administration protocol that separates authentication, authorization, and accounting.

Switching

Spanning Tree Protocol
A switching protocol that blocks loops while allowing redundant links.