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Computer Networking Basics

Learn computer networking with multiple-choice questions: the OSI reference model and TCP/IP, IP addressing and subnet calculations, ARP and routing, TCP and UDP, DNS, DHCP and TLS, transfer-time calculations, wireless LAN, firewalls and VPNs.

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Computer Networking Basics: key points

Even opening a single web page sets many mechanisms to work in turn: looking up an address from a name, choosing a route, delivering data reliably and protecting it with encryption. Computer networks organize these functions into layers, with each layer taking care of its own role only.

This topic covers the OSI reference model and the TCP/IP layers, MAC addresses and switches, IP addresses and subnet mask calculations, ARP and routing, TCP's three-way handshake, flow control and congestion control, and DNS, DHCP and TLS. It also covers calculating transfer times from bandwidth and latency, and the basics of wireless LAN, firewalls and VPNs, which helps with the network section of IT certification exams such as Japan's Fundamental Information Technology Engineer Examination.

All Computer Networking Basics questions (40)
  1. In the OSI reference model, which layer uses logical addresses such as IP addresses to relay packets between different networks until they reach their destination?
  2. In the four-layer TCP/IP model, which layer do TCP and UDP belong to?
  3. When a web page's data is sent, what happens on the sending side each time the data is passed from an upper layer to a lower layer?
  4. Which statement about MAC addresses is correct?
  5. Compared with a hub (repeater hub), which correctly describes how an L2 switch operates?
  6. On one L2 switch, VLANs are configured so that ports 1–4 are in VLAN 10 and ports 5–8 are in VLAN 20. Which statement about its operation is correct?
  7. Which correctly shortens the IPv6 address 2001:0db8:0000:0000:0000:0000:0000:0001 according to the abbreviation rules?
  8. A host has IP address 192.168.1.10 and subnet mask 255.255.255.0. What is the network address of the network it belongs to?
  9. In the network 192.168.5.0/26, how many IP addresses can be assigned to hosts (end devices or router interfaces)?
  10. A host has IP address 192.168.10.77/27. What is the broadcast address of its subnet?
  11. You want to fit 500 hosts in one subnet. Of the subnets that can do this, which is the smallest (has the longest prefix)?
  12. Which subnet mask is equivalent to /20 in CIDR notation?
  13. Which of the following IPv4 addresses falls within a private address range?
  14. Home broadband routers perform NAPT (IP masquerading), a form of NAT (Network Address Translation). What does NAPT make possible?
  15. Host A (192.168.1.10), inside a router performing NAPT whose outside global address is 198.51.100.7, connects from source port 50000 to port 443 on web server 203.0.113.5. The router translates the source to port 61000 on 198.51.100.7. What is the destination of the response packets the web server sends back?
  16. Which correctly describes the role of ARP?
  17. A host (192.168.1.10/24, default gateway 192.168.1.1) sends a packet to server 203.0.113.80 on a different network. With an empty ARP cache, whose MAC address does the host look up with ARP?
  18. A router's routing table has four routes: 0.0.0.0/0 (next hop router A), 10.0.0.0/8 (router B), 10.1.0.0/16 (router C) and 10.1.2.0/24 (router D). To which router is a packet for 10.1.5.9 sent?
  19. What is the role of the TTL (Time To Live) field in the IPv4 header?
  20. Which correctly describes dynamic routing?
  21. Which correctly describes UDP?
  22. In the three-way handshake that establishes a TCP connection, what is the correct order of the packets exchanged?
  23. A client makes a new HTTPS connection (TLS 1.3 over TCP) to a web server with a round-trip time (RTT) of 100 ms. Roughly how long does it take until the response to the first request starts to arrive? Assume DNS resolution is already done, and ignore server processing time and data transfer time.
  24. Which correctly pairs protocols with the standard port numbers their servers use?
  25. In TCP flow control, what is the purpose of the receiver advertising its window size (receive window) to the sender?
  26. A TCP connection has a fixed receive window of 65536 bytes and an RTT of 10 ms. The link bandwidth is ample and there is no loss. Which is closest to the upper limit on this connection's throughput?
  27. Which correctly describes the basic idea of TCP congestion control?
  28. What triggers a TCP sender to decide that a segment was lost and retransmit it?
  29. What is the main role of DNS (Domain Name System)?
  30. A full-service resolver (caching DNS server) with an empty cache looks up the IP address of www.example.com. In what order does it query servers?
  31. When a device obtains an IP address with DHCP, what is the correct order of messages?
  32. Which of the following does HTTPS (HTTP protected by TLS) protect?
  33. In TLS, why can a browser conclude that the server it connects to is not an impostor?
  34. Roughly how long does it take to send a 100 MB file over a 100 Mbps link? Ignore overhead such as headers and latency, and take 1 MB = 10^6 bytes.
  35. On 1 Gbps Ethernet, TCP data is sent continuously with IP packets filling the 1500-byte MTU. Taking the IPv4 header as 20 bytes, the TCP header as 20 bytes, the Ethernet header and FCS together as 18 bytes, and the preamble and inter-frame gap together as 20 bytes, which is closest to the upper limit on application-data throughput?
  36. Over a link with an RTT of 50 ms and a bandwidth of 100 Mbps, a request that returns a 10 KB response is repeated 100 times, each sent only after the previous response has been fully received. Which is closest to the total time? Take the time per request as the RTT plus the response transfer time (data size ÷ bandwidth), 1 KB = 1000 bytes, and ignore server processing time.
  37. What is the main reason wireless LAN uses CSMA/CA rather than CSMA/CD, which has been used in wired Ethernet?
  38. Which correctly describes a general difference between the 2.4 GHz and 5 GHz bands of wireless LAN?
  39. Which correctly describes the basic function of a (packet-filtering) firewall?
  40. Which correctly describes an internet VPN?

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