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CNPA: Network Fundamentals and Models

CNPA · Work through your exam, one topic at a time.

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Network Fundamentals and Models

This domain tests whether you can place any network function at the right layer and follow a payload from application data down to bits. Items ask you to name PDUs, map OSI to the TCP/IP model, choose TCP or UDP for a service, and reason about encapsulation overhead.

Layer duties and PDU names

  • Layer 1 (Physical): bits, media, signalling, pinouts, duplex.
  • Layer 2 (Data Link): frames, MAC addressing, FCS error detection, switching. No TTL, no logical addressing.
  • Layer 3 (Network): packets, logical addressing, path selection, TTL decrement, fragmentation.
  • Layer 4 (Transport): segments (TCP) or datagrams (UDP); port numbers give process-to-process delivery.
  • Layers 5-7 (Session, Presentation, Application) collapse into the TCP/IP Application layer alongside protocols such as HTTP, DNS and SMTP.

TCP/IP mapping: Network Access = OSI L1 + L2; Internet = L3; Transport = L4; Application = L5-L7.

Behaviours worth memorising

  • TCP: connection-oriented, three-way handshake (SYN, SYN-ACK, ACK), sequence and acknowledgement numbers, retransmission, sliding-window flow control, teardown with FIN or RST.
  • UDP: connectionless, no retransmission, 8-byte header, used by DNS queries, DHCP, TFTP, SNMP, NTP and RTP.
  • Devices: hub L1, switch and bridge L2, router L3, multilayer switch and firewall L3-L7.
  • Every layer adds its own header; default Ethernet MTU is 1500 bytes.

Worked example

A host sends 5,000 bytes of application data over Ethernet using IPv4 and TCP with no options. IPv4 header 20 bytes plus TCP header 20 bytes gives 40 bytes of overhead, so the TCP maximum segment size is 1500 - 40 = 1460. Dividing 5,000 by 1460 gives three full segments (4,380 bytes) plus one 620-byte segment: four packets. If a router on the path had a 1,400-byte MTU and the DF bit were set, it would drop the oversized packet and return ICMP type 3 code 4 (fragmentation needed). That is why "small pings work, large transfers hang" points at MTU or path MTU discovery.

Traps

  • ARP resolves an L3 address to an L2 address; questions often mislabel it as pure L2 or pure L3.
  • The OSI session layer is not the TCP connection; connections belong to L4.
  • UDP is not broken TCP; applications such as DNS add their own retry logic.
  • The FCS detects errors and discards the frame; it never corrects them.