Computer Networks Cheat Sheet

computer-networks Last verified: 28 Jul 2026

OSI model — 7 layers

Layer	Name	Function	Example protocol/device
7	Application	User-facing services	HTTP, FTP, SMTP, DNS
6	Presentation	Data translation, encryption, compression	SSL/TLS, JPEG
5	Session	Establishes/manages/terminates sessions	NetBIOS, RPC
4	Transport	End-to-end delivery, reliability	TCP, UDP
3	Network	Logical addressing, routing	IP, ICMP, Router
2	Data Link	Physical addressing, framing	Ethernet, MAC address, Switch
1	Physical	Raw bit transmission	Cables, Hub, radio signals

Mnemonic (top to bottom): All People Seem To Need Data Processing.

OSI vs TCP/IP model

OSI (7 layers)	TCP/IP (4 layers)
Application, Presentation, Session	Application
Transport	Transport
Network	Internet
Data Link, Physical	Network Access (Link)

OSI is a theoretical reference model; TCP/IP is the actual protocol suite the internet runs on. This distinction is a very common exam question.

PYQ

Which OSI layer is responsible for logical addressing and routing?

A) Data Link
B) Network
C) Transport
D) Session

Why: The Network layer (Layer 3) handles IP addressing and routing between networks. The Data Link layer (Layer 2) handles physical/MAC addressing within a single network segment.

PYQ

How many layers does the TCP/IP model have, compared to OSI's 7?

A) 3
B) 4
C) 5
D) 6

Why: TCP/IP combines OSI's Application+Presentation+Session into one Application layer, and Data Link+Physical into one Network Access layer — giving 4 layers total (some textbooks show 5 by splitting Network Access).

TCP vs UDP

Property	TCP	UDP
Connection	Connection-oriented (handshake required)	Connectionless
Reliability	Reliable — guarantees delivery, order	Unreliable — no delivery guarantee
Speed	Slower (overhead from reliability)	Faster (minimal overhead)
Use case	Web browsing, email, file transfer	Video streaming, DNS, gaming
Header size	20 bytes	8 bytes

TCP three-way handshake

Client                          Server
  |----------- SYN ------------>|   (Client requests connection)
  |<-------- SYN-ACK -----------|   (Server acknowledges + requests back)
  |----------- ACK ------------>|   (Client confirms — connection established)

Connection termination uses a 4-way handshake (FIN, ACK, FIN, ACK).

PYQ

Which protocol would a live video call most likely use, and why?

A) TCP — because it's reliable
B) UDP — because low latency matters more than guaranteed delivery
C) HTTP — because it's simple
D) FTP — because it transfers data fast

Why: Real-time video/audio prioritizes speed over reliability — a dropped packet causing a brief glitch is preferable to TCP's retransmission delay stalling the whole stream.

IP addressing

IPv4 address classes

Class	Range (first octet)	Default subnet mask	Typical use
A	1 – 126	255.0.0.0 (/8)	Very large networks
B	128 – 191	255.255.0.0 (/16)	Medium networks
C	192 – 223	255.255.255.0 (/24)	Small networks
D	224 – 239	—	Multicast
E	240 – 255	—	Reserved/experimental

127.x.x.x is reserved for loopback (localhost), not part of Class A’s usable range — a common trap.

CIDR notation

192.168.1.0/24
             ↑
             number of bits used for the NETWORK portion

/24 = 255.255.255.0 = 256 addresses total, 254 usable
      (1 reserved for network address, 1 for broadcast address)

Usable hosts = 2^(32 - prefix) - 2

PYQ

How many usable host addresses are there in a /24 subnet?

A) 256
B) 254
C) 255
D) 128

Why: A /24 gives 2^8 = 256 total addresses, but 2 are reserved — one for the network address, one for the broadcast address — leaving 254 usable for hosts.

Switching techniques

Technique	How it works	Trade-off
Circuit Switching	Dedicated path reserved for the entire session	Guaranteed bandwidth, but wastes idle capacity
Packet Switching	Data split into packets, routed independently	Efficient sharing, but variable delay (jitter)
Message Switching	Entire message stored and forwarded hop by hop	Simple, but high latency, needs storage at each node

The modern internet uses packet switching — this is a frequently tested fact.

Routing

Type	Strategy	Example protocol
Distance Vector	Each router shares its full routing table with neighbors; picks path with fewest hops	RIP
Link State	Each router builds a full map of the network topology, computes shortest path itself	OSPF

Distance Vector: "I'll tell you what I know, you figure out the rest"
                 — simpler, but slower to converge, prone to routing loops

Link State:      "I'll tell everyone exactly what I see"
                 — faster convergence, more overhead, no loops

PYQ

RIP (Routing Information Protocol) is an example of which routing approach?

A) Distance Vector
B) Link State
C) Static Routing
D) Hybrid Routing

Why: RIP uses the Bellman-Ford algorithm and hop count as its metric — the defining trait of Distance Vector routing. OSPF is the classic Link State example.

Error detection

Method	How it works	Detects
Parity bit	Adds 1 bit to make the count of 1s odd/even	Single-bit errors only
Checksum	Sum of data blocks, transmitted and re-verified	Most errors, some patterns missed
CRC (Cyclic Redundancy Check)	Polynomial division, remainder appended	Burst errors — most robust of the three

CRC is the standard for Ethernet frames precisely because it reliably catches burst errors that simple parity or checksums can miss.

PYQ

Which error detection method is most effective at catching burst errors?

A) Parity bit
B) Checksum
C) CRC
D) All are equally effective

Why: CRC uses polynomial division across the entire data block, making it far more robust against burst errors (consecutive corrupted bits) than a simple parity bit or checksum.

Network devices

Device	OSI Layer	Function
Hub	Physical (1)	Broadcasts to all ports — no intelligence
Switch	Data Link (2)	Forwards frames based on MAC address — learns port-MAC mapping
Router	Network (3)	Forwards packets between different networks, based on IP
Gateway	Any/all	Connects networks using different protocols entirely

PYQ

A switch makes forwarding decisions based on which address?

A) IP address
B) MAC address
C) Port number
D) Domain name

Why: A switch operates at the Data Link layer and forwards frames using MAC addresses, learning which device sits on which port. Routers, by contrast, use IP addresses (Network layer).

DNS, HTTP & common ports

Port	Protocol	Purpose
20/21	FTP	File transfer (data/control)
22	SSH	Secure remote login
23	Telnet	Unencrypted remote login
25	SMTP	Sending email
53	DNS	Domain name resolution
80	HTTP	Web traffic (unencrypted)
110	POP3	Retrieving email
443	HTTPS	Web traffic (encrypted)

DNS resolution flow:
  Browser → Local DNS cache → Recursive Resolver
  → Root Server → TLD Server (.com, .in, etc.)
  → Authoritative Name Server → IP address returned

PYQ

What is the default port number for HTTPS?

A) 80
B) 21
C) 443
D) 25

Why: Port 443 is the standard for HTTPS (encrypted web traffic). Port 80 is its unencrypted counterpart, HTTP.

CDAC C-CAT — top networking exam traps

Trap	Rule
OSI vs TCP/IP	OSI = 7-layer theoretical model. TCP/IP = 4-layer model that’s actually implemented on the internet.
TCP vs UDP	TCP = reliable, connection-oriented, slower. UDP = unreliable, connectionless, faster. Streaming/gaming favor UDP.
Switch vs Router	Switch works on MAC address (Layer 2). Router works on IP address (Layer 3).
Circuit vs Packet switching	The modern internet uses packet switching, NOT circuit switching.
Distance Vector vs Link State	RIP = Distance Vector (hop count). OSPF = Link State (full topology map).
/24 usable hosts	254, not 256 — 2 addresses are reserved (network + broadcast).
127.x.x.x	Reserved for loopback, NOT a usable Class A address.
CRC vs Checksum vs Parity	CRC catches burst errors best. Parity only catches single-bit errors.
Three-way handshake	SYN → SYN-ACK → ACK. Termination uses a separate 4-way FIN/ACK exchange.
Port 80 vs 443	80 = HTTP (unencrypted). 443 = HTTPS (encrypted, uses TLS/SSL).
DNS purpose	Translates domain names to IP addresses — it’s a lookup service, not a routing protocol.

PYQs are indicative of exam style, not guaranteed exact repeats.

Frequently Asked Questions

1 What is the difference between OSI and TCP/IP model?

OSI has 7 layers while TCP/IP has 4 layers. OSI is a theoretical model, TCP/IP is the practical implementation used on the internet.

2 Which layer handles routing in OSI model?

The Network Layer (Layer 3) handles routing, logical addressing, and path determination between source and destination.