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2.1 - CompTIA A+ Core 1

Exam Objective 2.1: Compare and contrast Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) ports, protocols, and their purposes.

Ports and protocols

  • 20/21 – File Transfer Protocol (FTP)

    • Runs on TCP port 20 for data transfers and TCP port 21 for controlling data transfers

    • Function: Transfers files between systems.

    • Typically requires a username/password combination to authenticate - some systems allow anonymous logins, however.

    • Includes file management functionalities, such as listing, adding, deleting, etc.

  • 22 – Secure Shell (SSH)

    • Runs on TCP port 22

    • Function: Allows encrypted communications between a client, and the console/CLI for a remote device

    • Uses a text-based CLI screen; looks exactly the same as Telnet

  • 23 – Telnet

    • Runs on TCP port 23

    • Function: Allows non-encrypted communications between a client console/CLI and a remote device

  • 25 – Simple Mail Transfer Protocol (SMTP)

    • Runs on TCP port 25

    • Function: Used to send client mail to a mail server (client → server), and server ←→ server mail communications

    • Typically used on mobile devices and email clients.

  • 53 – Domain Name System (DNS)

    • Runs on UDP port 53

    • Function: Resolves (converts) domain names to IP addresses

    • Highly critical resources; multiple IP addresses (webservers) are used for a website, so memorizing IP addresses (sans DNS), is a highly cumbersome task.

  • 67/68 – Dynamic Host Configuration Protocol (DHCP)

    • Runs on UDP ports 67/68

    • Function: Automatically assigns IP addresses to network devices.

      • Takes IP addresses from a real-time IP pool. Each endpoint receives an IP lease that must be renewed at set intervals (such as devices entering/leaving a network).

      • IP addresses can also be assigned by MAC addresses - this is managed via the DHCP server

    • Requires a DHCP server, but this functionality is typically built into routers.

  • 80 – Hypertext Transfer Protocol (HTTP)

    • Runs on TCP port 80

    • Used for client-webserver communications

  • 443 – Hypertext Transfer Protocol Secure (HTTPS)

    • Runs on TCP port 443

    • Used for encrypted client-webserver communications

  • 110 – Post Office Protocol 3 (POP3)

    • Runs on TCP port 110

    • Function: Basic (i.e., single client), server → client mail transfer.

  • 137/139 – Network Basic Input/ Output System (NetBIOS)/ NetBIOS over TCP/IP (NetBT)

    • Protocol used by SMB to communicate

    • Runs on UDP port 137 for NetBIOS name services (nbname)

    • Runs on TCP port 139 for NetBIOS session services (nbsession)

    • NetBT/NetBIOS-less:

      • Runs on TCP port 445

      • Function: Direct SMB communication between devices over TCP without NetBIOS

  • 143 – Internet Mail Access Protocol 4 (IMAP/IMAP4)

    • Runs on TCP port 143

    • Function: Multi-client (multi-device) email retrieval; includes an email inbox for better management

  • 161/162 – Simple Network Management Protocol (SNMP)

    • Function: Allows network admin to query statistics/information from network devices.

    • Uses UDP port 161 for queries

    • Uses UDP port 162 for network traps (information sent from network devices)

    • SNMP versions:

      • v1: Structured tables, plaintext data

      • v2: Bulk data transfers, data type enhancements, still plaintext

      • v3: Message integrity, authentication (i.e., logins), encrypted data

  • 389 – Lightweight Directory Access Protocol (LDAP)

    • Function: Used to query network directories, and retrieve information from network directories

    • Uses TCP port 389

    • Commonly used in Microsoft Active Directory

  • 443 – Hypertext Transfer Protocol Secure (HTTPS)

  • 445 – Server Message Block (SMB)/Common Internet File System (CIFS)

    • Runs on TCP port 445

    • Function: Used for file/printer sharing

  • 3389 – Remote Desktop Protocol (RDP)

    • Function: Standard port used by Windows (on many different versions) for remote computer sharing

    • Runs on TCP port 3389

    • Can be used to connect/take over an entire system, or just an application

    • RDP servers are Windows machines, but RDP clients (machines attempting to connect via RDP) can be non-Windows devices

  • TCP vs. UDP

    • Connectionless

      • DHCP

      • Trivial File Transfer Protocol (TFTP)

    • Connection-oriented

      • HTTPS

      • SSH