1.5 · TCP/IP Basics

Goal: explain TCP and IP separately, why they matter, and the role of protocols in general.

What a "protocol" is

A communication protocol is an agreed set of rules that two devices follow to exchange data successfully — like a language agreed in advance.

Without protocols:

• The receiver wouldn't know where one message ends and the next begins.
• Errors would go undetected.
• Lost packets would never be retransmitted.

TCP / IP — two pieces, one suite

Protocol	Role
IP (Internet Protocol)	Provides addressing and routing — gets each packet to the right machine
TCP (Transmission Control Protocol)	Provides reliable, ordered, error-checked delivery between two programs (port-to-port)

Together they are the "TCP/IP" protocol suite that the Internet runs on.

Analogy · Postcards and tracked mail

• IP is like dropping postcards in a mailbox — they may arrive in any order, some may get lost.
• TCP is like Hong Kong Post's tracked mail service — confirms delivery, re-sends if lost, delivers in order.

How they fit together (simplified)

Application data ── TCP ── chops into segments, numbers them
                              │
                              ▼
                         IP ── adds source/dest IP, sends as packets
                              │
                              ▼
                  Network hardware (NIC, switches, routers)

At the destination, IP delivers packets to the correct host; TCP reassembles them in order and asks for resends if needed.

Other protocols you should recognise

Protocol	Stands for	Use
HTTP	HyperText Transfer Protocol	Browsing (port 80)
HTTPS	HTTP Secure	Browsing with TLS encryption (port 443)
FTP	File Transfer Protocol	File transfer (port 21)
SMTP	Simple Mail Transfer Protocol	Sending email (port 25)
POP3 / IMAP	Post Office / Internet Message Access	Receiving email
DNS	Domain Name System	Name → IP resolution
DHCP	Dynamic Host Configuration Protocol	Auto-assign IP
UDP	User Datagram Protocol	Fast, no-guarantee alternative to TCP (used for video calls, DNS queries)

Why TCP and UDP coexist

Aspect	TCP	UDP
Reliability	Yes (resends lost packets)	No
Ordering	Yes	No
Speed overhead	Higher	Lower
Used for	Web, email, file transfer	Video / voice calls, online games, DNS

Real-time video and gaming prefer UDP — late packets are useless anyway; better to skip and stay current.

Exam-style question

Q (4 marks): Explain the difference between TCP and IP, and why a protocol is needed for two computers to communicate.

Sample answer:

• IP handles addressing and routing — every packet carries source and destination IP addresses, and routers forward packets toward the destination.
• TCP ensures reliable, ordered delivery between programs by numbering segments, acknowledging receipt, and re-sending lost ones.
• A protocol is a set of agreed rules; without it, two computers (possibly from different vendors and OSes) would have no common understanding of how data is formatted, addressed, or acknowledged, making reliable communication impossible.

Key takeaways

• IP = addressing & routing.
• TCP = reliable, ordered byte stream.
• UDP = fast, no-guarantee alternative.
• Protocols are agreed rules — without them, networking does not work.

Chapter 1 wrap-up

You've finished networking fundamentals. Self-test:

• LAN vs WAN — coverage, owner, example?
• IPv4 vs IPv6 — bits, notation, reason for transition?
• Switch vs router — same LAN vs between LANs?
• TCP vs UDP — when to use each?

➡️ Next chapter: 2 · Internet Services & Applications