Cisco CCNA Preparation · lesson 2 of 36 · 25 min
Why Models Matter
Networking models provide a common language for describing how data moves from one device to another. The two models you must know for the CCNA are the OSI model (7 layers, conceptual) and the TCP/IP model (4 layers, practical). The OSI model is used for troubleshooting and discussion; TCP/IP is what actually runs on networks today.
The OSI Model — 7 Layers
Memory trick
Top-down: All People Seem To Need Data Processing (Application → Physical)Bottom-up: Please Do Not Throw Sausage Pizza Away (Physical → Application)
The TCP/IP Model — 4 Layers
TCP vs UDP
TCP 3-way handshake
SYN → Client sends SYN to serverSYN-ACK → Server replies with SYN-ACKACK → Client sends ACK — connection established
PDU Names by Layer
Flashcard Drill
OSI Layer 7 Application — user-facing services (HTTP, DNS, SMTP, DHCP). OSI Layer 4 Transport — end-to-end delivery, segmentation, TCP and UDP. OSI Layer 3 Network — logical addressing and routing, IP, routers. OSI Layer 2 Data Link — MAC addressing, framing, switches. OSI Layer 1 Physical — bits on the wire, cables, hubs, NICs. TCP Connection-oriented, reliable, ordered delivery — uses 3-way handshake (SYN/SYN-ACK/ACK). UDP Connectionless, best-effort, fast — used for DNS, VoIP, streaming. Segment Layer 4 PDU (TCP). Datagram for UDP. Packet Layer 3 PDU — contains source and destination IP addresses. Frame Layer 2 PDU — contains source and destination MAC addresses.
Knowledge Check
OSI & TCP/IP — 10 Questions
At which OSI layer does a router operate?
- Layer 1
- Layer 2
- Layer 3
- Layer 4
Answer: Layer 3
Routers operate at Layer 3 (Network) and use IP addresses to forward packets.
What is the PDU name at OSI Layer 2?
- Bit
- Packet
- Frame
- Segment
Answer: Frame
Layer 2 (Data Link) PDUs are called frames — they contain MAC addresses.
Which protocol provides reliable, ordered, connection-oriented delivery?
- UDP
- IP
- TCP
- ICMP
Answer: TCP
TCP is connection-oriented and provides reliable, ordered delivery with ACKs and retransmission.
The TCP connection establishment process uses a ___-way handshake.
Answer: 3
TCP uses a 3-way handshake: SYN → SYN-ACK → ACK.
DNS uses which transport protocol?
- TCP only
- UDP (primarily)
- Both equally
- Neither
Answer: UDP (primarily)
DNS primarily uses UDP port 53 for queries. TCP is used for zone transfers and large responses.
Which OSI layer handles encryption and data formatting?
- Layer 5 Session
- Layer 6 Presentation
- Layer 7 Application
- Layer 4 Transport
Answer: Layer 6 Presentation
Layer 6 (Presentation) handles encryption, compression, and data format translation.
UDP guarantees delivery and orders packets.
- True
- False
Answer: False
UDP is connectionless and best-effort — no guaranteed delivery or ordering.
The TCP/IP Internet layer maps to which OSI layer?
- OSI Layer 2
- OSI Layer 3
- OSI Layer 4
- OSI Layer 5
Answer: OSI Layer 3
The TCP/IP Internet layer (IP, ICMP) maps to OSI Layer 3 (Network).
Which OSI layer does a switch primarily operate at?
- Layer 1
- Layer 2
- Layer 3
- Layer 4
Answer: Layer 2
Switches operate at Layer 2 (Data Link) and forward frames based on MAC addresses.
A Layer 3 PDU is called a _____.
Answer: packet
Layer 3 (Network) PDUs are called packets — they contain source and destination IP addresses.
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