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Networking: Devices, Topologies & Protocols

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Protocols and port numbers; TCP vs UDP

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⏱ 5 min read🧩 4 question types🎯 17 practice Q
The idea in one minute
ProtocolPortPurpose
HTTP80Web pages (plain)
HTTPS443Web over TLS/SSL encryption
FTP21 (control) / 20 (data)File transfer
SSH22Secure remote login (encrypted successor of Telnet)
Telnet23Plain-text remote login (insecure)
SMTP25Sending e-mail
DNS53Name-to-IP resolution
DHCP67/68Automatic IP assignment
POP3110Receive e-mail (download)
IMAP143Receive e-mail (sync)
SNMP161Network management/monitoring
RDP3389Windows remote desktop

TCP (Transmission Control Protocol) - connection-oriented, 3-way handshake (SYN, SYN-ACK, ACK), guaranteed ordered delivery: web, e-mail, file transfer. UDP (User Datagram Protocol) - connectionless, no delivery guarantee, faster/lower overhead: DNS queries, DHCP, video/audio streaming, VoIP, online gaming. Standards: Ethernet = IEEE 802.3, Wi-Fi = 802.11, Bluetooth = 802.15.

01

Ports — numbered doors on a machine

While the IP address finds the computer, the port number finds the service (door) on it. The classic exam set:

ProtocolPortJob
HTTP80web pages (plain)
HTTPS443web pages encrypted (TLS/SSL)
FTP21 (control; 20 = data)file transfer
SSH22secure remote login
Telnet23remote login (NOT encrypted)
SMTP25sending e-mail
DNS53name to IP resolution
POP3110receiving e-mail (download)
IMAP143receiving e-mail (sync)

Anchor pairs: 80/443 web, 21/23/25 climbing odd numbers (FTP, Telnet, SMTP), 53 DNS, 110/143 mail-in.

02

TCP vs UDP — the two delivery styles

TCP (Transmission Control Protocol)UDP (User Datagram Protocol)
Connectionconnection-oriented (handshake first)connectionless (just send)
Reliabilityguaranteed, ordered, error-checkedno guarantee — fast but unsafe
Speedslowerfaster
Used byweb (HTTP/HTTPS), e-mail (SMTP), file transfer (FTP)live streaming, gaming, VoIP, DNS queries, DHCP

Memory hook: TCP = Tracked, Careful, Precise; UDP = Untracked, Dashes. Live cricket streaming chooses UDP — one lost frame is better than a paused match.

03

The protocols behind one web visit

  1. DNS (UDP 53) turns the name into an IP.
  2. TCP opens the connection.
  3. HTTPS (TCP 443) requests and delivers the page encrypted. For e-mail: SMTP sends, POP3/IMAP receive — over TCP.
04

Full forms worth memorising

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; TCP Transmission Control Protocol; UDP User Datagram Protocol; DNS Domain Name System; DHCP Dynamic Host Configuration Protocol; SSH Secure Shell.

05

Reading a protocol question in the exam

Ask first: is the question about a direction (send mail = SMTP, receive = POP3/IMAP), a number (port), a style (reliable or fast), or a full form? For port questions, if you cannot recall the exact number, eliminate by neighbourhood — 21-22-23 are the file/remote trio, 25 is the mail sender, 80/443 the web pair, 110/143 the mail-in pair. For TCP-or-UDP questions, ask what a lost packet would cost: a corrupted file (use TCP) or one blurred frame (UDP is fine).

06

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', 'port of SMTP/FTP/DNS/POP3', or a matched pair with one number shifted.

Method

  1. Anchors: HTTP 80, HTTPS 443, FTP 21, SSH 22, Telnet 23, SMTP 25, DNS 53, POP3 110, IMAP 143.

  2. The classic trap: 443 for plain HTTP or 80 for HTTPS.

  3. Remember 21-22-23 climbing (FTP, SSH, Telnet) and 25 as the mail sender's door.

Try this

HTTPS uses which port number by default?

Show solution

443 — plain HTTP uses 80; the S (secure) doubles the number in exam memory: 80 becomes 443.

Type 2very common4 practice Q

TCP vs UDP selection

How to spot it:

'Which protocol is used for live streaming / file transfer', 'which is connection-oriented', reliability-vs-speed statements.

Method

  1. TCP: connection-oriented, reliable, ordered — web, e-mail, FTP.

  2. UDP: connectionless, fast, no delivery guarantee — live streaming, gaming, VoIP, DNS queries.

  3. The classic exam answer: live video streaming = UDP; downloading a file = TCP.

Try this

Which transport protocol suits a live cricket stream?

Show solution

UDP — speed matters more than perfect delivery; TCP's retransmissions would only pause the match.

Type 3common3 practice Q

Protocol purpose identification

How to spot it:

'Which protocol transfers files', 'which protocol sends e-mail', 'which service resolves names' — purpose matching across the famous protocols.

Method

  1. File transfer = FTP; send mail = SMTP; receive mail = POP3/IMAP; web pages = HTTP/HTTPS; names = DNS; remote login = Telnet (plain) / SSH (secure).

  2. FTP has two ports: 21 control, 20 data.

  3. DHCP assigns IP addresses automatically.

Try this

Which protocol is used to transfer files between computers?

Show solution

FTP (File Transfer Protocol) — control on port 21, data on port 20.

Type 4common3 practice Q

Protocol full forms and security

How to spot it:

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

Method

  1. FTP = File Transfer Protocol; TCP = Transmission Control Protocol; UDP = User Datagram Protocol; DHCP = Dynamic Host Configuration Protocol; SSH = Secure Shell.

  2. Telnet (23) sends everything in plaintext; SSH (22) is the encrypted replacement.

  3. HTTPS = HTTP over TLS/SSL.

Try this

Which protocol provides an ENCRYPTED remote login?

Show solution

SSH (Secure Shell), port 22 — Telnet on port 23 sends passwords as plain text.

07

Shortcuts that save time

⚡ Port anchors

Memorise six: 80 HTTP, 443 HTTPS, 25 SMTP, 53 DNS, 110 POP3, 21 FTP - these cover most CKT options. Then '22 SSH, 23 Telnet' (Telnet = the insecure 23).

Example

Default port of HTTPS?

Show solution
⚡ TCP = Tracked, UDP = Untracked

TCP Tracks every packet (reliable, slower - web/mail/files); UDP is Unreliable but quick (streaming, gaming, DNS). 'Guaranteed delivery' in the question = TCP.

Example

Live cricket video streaming mainly uses?

Show solution

UDP (speed over retransmission).

08

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Giving HTTP the 443 port - 443 is HTTPS; plain HTTP is 80.

Mistake 02

Choosing TCP for live streaming in the classic question - the intended answer is UDP (speed first).

Mistake 03

Saying Telnet is secure - SSH (22) is the encrypted one; Telnet (23) sends everything in clear text.

09

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 = reliable, connection-oriented (web, mail, FTP); UDP = fast, connectionless (streaming, games, VoIP).

  • Telnet is plaintext; SSH (22) is its encrypted replacement.

  • HTTPS = HTTP + TLS/SSL encryption.

10

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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