Networking: Devices, Topologies & Protocols

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Protocols and port numbers; TCP vs UDP

⏱ 4 min read🧩 6 question types🎯 17 practice Q

The idea in one minute

The IP address finds the computer. The port number finds the service on it. Learn the main ports: HTTP 80, HTTPS 443, FTP 21, SSH 22, Telnet 23, SMTP 25, DNS 53, POP3 110, IMAP 143. TCP is reliable and connection-oriented. UDP is fast and connectionless.

Ports, numbered doors on a machine

The IP address finds the computer. The port number finds the service on it. Learn this classic set.

Protocol	Port	Job
HTTP	80	web pages (plain)
HTTPS	443	web pages, encrypted with TLS/SSL
FTP	21 (control), 20 (data)	file transfer
SSH	22	secure remote login
Telnet	23	remote login, not encrypted
SMTP	25	sending e-mail
DNS	53	name to IP resolution
DHCP	67 / 68	automatic IP assignment
POP3	110	receiving e-mail (download)
IMAP	143	receiving e-mail (sync)
SNMP	161	network management
RDP	3389	Windows remote desktop

Tip: 21, 22, 23 climb in order: FTP, SSH, Telnet. Then 25 is SMTP. 80 and 443 are the web pair. 110 and 143 bring mail in.

TCP vs UDP

	TCP	UDP
Full form	Transmission Control Protocol	User Datagram Protocol
Connection	connection-oriented (handshake first)	connectionless (just send)
Delivery	guaranteed and ordered	no guarantee
Speed	slower	faster, low overhead
Used by	web, e-mail, file transfer	DNS queries, DHCP, streaming, VoIP, gaming

The TCP handshake has three steps: SYN, SYN-ACK, ACK.

Example: Live cricket streaming uses UDP. One lost frame is better than a paused match.

One web visit, step by step

DNS turns the name into an IP address.
TCP opens the connection.
HTTPS (port 443) asks for the page and delivers it encrypted.

For e-mail, SMTP sends. POP3 or IMAP receive.

Reading a protocol question

Decide what the question asks. Then use the matching tool.

Direction: send mail is SMTP. Receive mail is POP3 or IMAP.
Number: recall the port anchors. If unsure, eliminate by neighbourhood.
Style: reliable means TCP. Fast means UDP.
Full form: spell it out from the name.

Tip: For TCP or UDP, ask what a lost packet costs. A corrupted file is bad, so use TCP. One blurred video frame is fine, so use UDP.

Full forms to memorise

HTTP: HyperText Transfer Protocol. HTTPS: secure HTTP over TLS/SSL.
FTP: File Transfer Protocol. SMTP: Simple Mail Transfer Protocol.
POP: Post Office Protocol. IMAP: Internet Message Access Protocol.
DNS: Domain Name System. DHCP: Dynamic Host Configuration Protocol. SSH: Secure Shell.

Watch: Telnet sends everything in plain text. SSH (22) is the encrypted replacement.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common4 practice Q

Port-number matching

How to spot it:

HTTPS works on which port, the port of SMTP, FTP, DNS or POP3, or a matched pair with one number shifted.

Method

Recall the anchors: HTTP 80, HTTPS 443, FTP 21, SSH 22, Telnet 23, SMTP 25, DNS 53, POP3 110, IMAP 143.
Watch for 443 given to HTTP or 80 given to HTTPS.
Use the climbing run 21, 22, 23.
Eliminate by neighbourhood if unsure.

Why it works:

Port numbers are fixed facts, so anchors beat guessing.

Try this

HTTPS uses which port number by default?

Show solution

HTTPS is secure web traffic.
The port is 443. Plain HTTP uses 80.

Answer

443

Type 2very common4 practice Q

TCP vs UDP selection

How to spot it:

Which protocol is used for live streaming or file transfer, which is connection-oriented, or reliability-versus-speed statements.

Method

Ask what a lost packet would cost.
A corrupted file is costly: TCP.
One blurred frame is fine: UDP.
Live streaming, gaming, VoIP, DNS queries: UDP.

Why it works:

Reliability needs TCP. Speed needs UDP.

Try this

Which transport protocol suits a live cricket stream?

Show solution

Speed matters more than perfect delivery.
TCP retransmission would only pause the match.

Answer

UDP

Type 3common3 practice Q

Protocol purpose identification

How to spot it:

Which protocol transfers files, sends e-mail or resolves names. It is purpose matching.

Method

File transfer: FTP. Send mail: SMTP.
Receive mail: POP3 or IMAP. Web pages: HTTP or HTTPS.
Names to IPs: DNS. Remote login: Telnet (plain) or SSH (secure).
DHCP assigns IP addresses.

Why it works:

Each protocol was built for one purpose.

Try this

Which protocol is used to transfer files between computers?

Show solution

File transfer is the purpose.
FTP uses port 21 for control and 20 for data.

Answer

FTP (File Transfer Protocol)

Type 4common3 practice Q

Protocol full forms and security

How to spot it:

Full form of FTP, TCP, UDP or DHCP, which remote-login protocol is secure, or Telnet-versus-SSH statements.

Method

Spell out the full form from the name.
Telnet (23) sends plain text.
SSH (22) is the encrypted replacement.
HTTPS is HTTP over TLS/SSL.

Why it works:

The S in SSH and HTTPS means secure.

Try this

Which protocol provides an encrypted remote login?

Show solution

Telnet on port 23 sends passwords as plain text.
SSH is the encrypted one.

Answer

SSH (Secure Shell), port 22

Type 5common

Mail protocol direction

How to spot it:

Which protocol sends mail, which downloads it, or which keeps mail in sync across devices.

Method

Sending: SMTP (port 25).
Downloading: POP3 (port 110).
Syncing: IMAP (port 143).
All three run over TCP.

Why it works:

Sending and receiving use different protocols.

Try this

Which protocol is used to send e-mail?

Show solution

Sending is the direction asked.
SMTP is the sender; POP3 and IMAP receive.

Answer

SMTP (port 25)

Type 6occasional

Less common ports and services

How to spot it:

Ports 67/68, 161 or 3389, or which service uses them.

Method

DHCP uses 67 and 68.
SNMP (network management) uses 161.
RDP (Windows remote desktop) uses 3389.
FTP data port is 20.

Why it works:

These ports appear less often but are fixed facts.

Try this

Windows Remote Desktop uses which port?

Show solution

RDP means Remote Desktop Protocol.
Its port is 3389.

Answer

3389

Shortcuts that save time

⚡ Port anchors

Memorise six first: 80 HTTP, 443 HTTPS, 25 SMTP, 53 DNS, 110 POP3, 21 FTP. Then 22 SSH and 23 Telnet. Telnet is the insecure one.

Example

What is the default port of HTTPS?

Show solution

HTTPS is the secure web protocol.
Its port is 443.

Answer

443

⚡ TCP tracks, UDP does not

TCP tracks every packet, so it is reliable but slower. UDP is unreliable but quick. 'Guaranteed delivery' in a question means TCP.

Example

Live cricket video streaming mainly uses which protocol?

Show solution

Speed matters more than perfect delivery.
Retransmission would pause the match.

Answer

UDP

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Giving HTTP the port 443.

443 is HTTPS. Plain HTTP is 80.

Mistake 02

Choosing TCP for live streaming.

The expected answer is UDP, because speed comes first.

Mistake 03

Saying Telnet is secure.

SSH (22) is encrypted. Telnet (23) sends plain text.

Mistake 04

Using SMTP to receive mail.

SMTP sends mail. POP3 and IMAP receive it.

Quick revision

Read this the night before the exam.

HTTP 80, HTTPS 443, FTP 21, SSH 22, Telnet 23, SMTP 25, DNS 53, POP3 110, IMAP 143.
TCP is reliable and connection-oriented (web, mail, FTP). UDP is fast and connectionless (streaming, games, VoIP).
Telnet is plain text. SSH (22) is its encrypted replacement.
HTTPS is HTTP plus TLS/SSL encryption.
SMTP sends mail. POP3 and IMAP receive mail.
DNS resolves names to IP addresses. DHCP assigns IP addresses.

Practice: 17 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 5 min · wrong answers go to your mistake notebook automatically.

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