Ports 0–99

Port 7 — Echo Protocol

At a Glance

• Transport: TCP/UDP

• Common service: Echo

• Main purpose: Returns data back to sender

• Status: Official, legacy

• Related ports: None

What It Does

Returns received data unchanged. The legacy service was designed for testing and measurement, but it is rarely enabled today and can be abused in reflection or amplification attacks.

ELI Explains Shout into a canyon, hear your voice come back. Echo does this for computer messages — a "testing, can you hear me?" check.

A 10-Year-Old's Example Tin-can telephone: say "hello," friend says "hello" back. Echo proves the string works.

Remember This Legacy network mirror — disable on production systems.

Security Note Can be abused in DDoS amplification attacks.

Port 20 — FTP Data

At a Glance

• Transport: TCP

• Common service: FTP Data

• Main purpose: Transfers files during FTP sessions

• Status: Official; legacy and unencrypted

• Related ports: 21, 989

What It Does

In active FTP, the server normally uses TCP port 20 as the source of the data connection. The control session remains on port 21. Passive FTP instead negotiates a server-side data port.

ELI Explains Order food at the front counter (21), pick up at the back door (20). Two doors so talking and delivering don't block each other.

A 10-Year-Old's Example Call pizza on one line to order. They call back on a second line to confirm. Two calls, two jobs.

Remember This Port 20 is associated with active-mode FTP data. For secure transfer, use SFTP on port 22 or a correctly configured FTPS deployment.

Port 21 — FTP Control

At a Glance

• Transport: TCP

• Common service: FTP Control

• Main purpose: Commands, login, session management

• Status: Official; legacy and unencrypted

• Related ports: 20, 989, 990

What It Does

The FTP command channel — login, directory navigation, transfer setup. Credentials and commands travel in plaintext. Every FTP session begins here.

ELI Explains Library front desk: tell the librarian which book, they check your card, someone brings it through a side door. Talking and delivery are separate.

A 10-Year-Old's Example Game store counter: "I want Mario Kart." Clerk checks membership. Someone brings it from the back. The counter is port 21.

Remember This FTP control on port 21 is plaintext unless upgraded with explicit FTPS. SFTP uses SSH on port 22; implicit FTPS on port 990 is a legacy deployment model.

Port 22 — SSH

At a Glance

• Transport: TCP

• Common service: Secure Shell

• Main purpose: Encrypted remote access and file transfer

• Status: Official, current

• Related ports: 23, 2222

What It Does

Encrypted remote terminal, secure file transfer (SFTP), and port forwarding. Replaces insecure Telnet. Essential for system administration.

ELI Explains A secret tunnel between your computer and one far away. Type commands as if there — nobody spies. Walkie-talkie with unbreakable code.

A 10-Year-Old's Example Magic notebooks: whatever you write appears in your friend's, only you two can read. SSH is that magic notebook.

Remember This SSH encrypts remote access — always use instead of Telnet.

Port 23 — Telnet

At a Glance

• Transport: TCP

• Common service: Telnet

• Main purpose: Unencrypted remote terminal

• Status: Official, discouraged

• Related ports: 22

What It Does

Remote command-line access transmitting everything in plaintext — including passwords. Once standard, now strongly discouraged.

ELI Explains Yelling instructions across a crowded room. Everyone hears your password. SSH whispers through a secure tube instead.

A 10-Year-Old's Example Shout game commands across the living room. Brother hears every cheat code. Telnet: anyone in between reads everything.

Remember This Telnet sends passwords in plaintext — use SSH on port 22.

Port 25 — SMTP

At a Glance

• Transport: TCP

• Common service: Simple Mail Transfer Protocol

• Main purpose: Routes email between mail servers

• Status: Official, current

• Related ports: 465, 587

What It Does

Server-to-server email relay — the backbone of global email. Often blocked by ISPs to prevent spam. Use port 587 for client submission.

ELI Explains The postal sorting center where mail trucks exchange bags between cities. For post offices talking to post offices. Use 587 for personal mail.

A 10-Year-Old's Example School loading dock where trucks exchange packages with other schools. Students use the office counter (587), not the dock.

Remember This Port 25 relays email between servers. Use port 587 to send from your client.

Port 53 — DNS

At a Glance

• Transport: TCP/UDP

• Common service: Domain Name System

• Main purpose: Translates names to IP addresses

• Status: Official, current

• Related ports: 853

What It Does

DNS translates names and other records. UDP handles many ordinary queries; TCP is used for zone transfers and when a reliable stream is needed, including some truncated or large responses.

ELI Explains The internet's directory helper. People remember names, while networks route using IP addresses. Ask for a name, and DNS returns the applicable record, like a directory finding a number.

A 10-Year-Old's Example Want to call Alex but don't know the number. School directory looks up "Alex" and gives you the number. DNS is that directory.

Remember This DNS translates names to IPs — the internet's phonebook. Standard DNS is unencrypted.

Port 67 — DHCP Server

At a Glance

• Transport: UDP

• Common service: DHCP Server

• Main purpose: Assigns IP addresses to clients

• Status: Official, current

• Related ports: 68

What It Does

Listens for requests from devices joining a network. Assigns IP address, subnet mask, gateway, and DNS servers automatically.

ELI Explains Hotel front desk: guest arrives, desk assigns a room number and hands over a key. DHCP prevents two guests in the same room.

A 10-Year-Old's Example Camp check-in: counselor finds you an empty bunk, writes your name next to bunk 14. DHCP is that counselor.

Remember This Port 67 is the DHCP server — it hands out IP addresses.

Port 68 — DHCP Client

At a Glance

• Transport: UDP

• Common service: DHCP Client

• Main purpose: Requests network configuration

• Status: Official, current

• Related ports: 67

What It Does

Client side of DHCP. Devices broadcast discovery from here when joining a network. Used for lease renewal after assignment.

ELI Explains Raising your hand: "I'm here! Who can give me an address?" Server on 67 hears and responds. "Can I keep this address?" later.

A 10-Year-Old's Example Registration desk: "Hi, I'm new — badge for me?" Your question (68), their answer (67).

Remember This Port 68 is the DHCP client — devices broadcast from here for an address.

Port 69 — TFTP

At a Glance

• Transport: UDP

• Common service: Trivial File Transfer Protocol

• Main purpose: Simple file transfers, no authentication

• Status: Official; specialized and unencrypted

• Related ports: 21, 22

What It Does

Transfers files with minimal overhead, no authentication or encryption. Used for router firmware updates and config transfers on trusted networks.

ELI Explains Community bulletin board — anyone pins up or takes down notes without signing in. Fast, simple, no locks. Only inside trusted networks.

A 10-Year-Old's Example Classroom corkboard: anyone pins a drawing or takes one. No sign-in, just quick sharing. TFTP is that corkboard.

Remember This TFTP: fast file transfers, no auth. Restrict to trusted internal networks.

Security Note No encryption or authentication — never expose externally.

Port 80 — HTTP

At a Glance

• Transport: TCP

• Common service: Hypertext Transfer Protocol

• Main purpose: Transfers unencrypted web pages

• Status: Official, current

• Related ports: 443, 8080

What It Does

Delivers web pages in plaintext. The foundation of the World Wide Web. Anyone on the network can read unencrypted traffic.

ELI Explains Sending a postcard — everyone handling it can read it. Fine for public news, terrible for passwords. HTTPS (443) is a sealed envelope.

A 10-Year-Old's Example "Show me the newspaper" on a postcard. Anyone reads it. Reply is another postcard. HTTP is those postcards.

Remember This Port 80 delivers unencrypted web pages. Use HTTPS (443) for anything private.

Port 88 — Kerberos

At a Glance

• Transport: TCP/UDP

• Common service: Kerberos Authentication

• Main purpose: Secure authentication with encrypted tickets

• Status: Official, current

• Related ports: 464

What It Does

Uses cryptographic tickets so services do not receive a user's password on every request. A password or another credential may still be used during initial authentication. Kerberos is the default authentication protocol in Active Directory environments.

ELI Explains Concert wristband: show ID once, get wristband. Flash it at every stage — no ID needed again. Kerberos gives your computer an encrypted wristband.

A 10-Year-Old's Example Family pass at amusement park — everyone gets a hand stamp. Show stamp at every ride all day. Kerberos is that stamp.

Remember This Kerberos uses encrypted tickets for authentication — default in Active Directory.