Computer Networks and the Internet
Computer Networks and the Internet tests layered models, encapsulation, addressing and the protocols behind everyday services.
Core rules
- OSI layers: physical, data link, network, transport, session, presentation, application. TCP/IP collapses these into network access, internet, transport, application.
- Encapsulation: data → TCP segment → IP packet → Ethernet frame → bits; each layer adds its own header.
- TCP is connection-oriented and reliable (three-way handshake, retransmission, ordering); UDP is connectionless and cheap - used by DNS, streaming, online games.
- Well-known ports: HTTP 80, HTTPS 443, DNS 53, SMTP 25, SSH 22.
- Private ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; NAT maps them to a public address.
- A switch forwards frames by MAC address (layer 2); a router forwards packets by IP address (layer 3). DHCP assigns addresses; DNS resolves names.
Worked example
Split 192.168.10.0/26. The mask is 255.255.255.192 and the block size is 64, so subnets start at .0, .64, .128 and .192. For the first: network address .0, usable hosts .1–.62, broadcast .63, giving 62 hosts. Reasoning: 32 − 26 = 6 host bits, 2^6 = 64 addresses, minus the network and broadcast addresses.
Traps
- Counting 64 hosts instead of 62, or forgetting the broadcast address.
- Saying a switch routes traffic between subnets - that is a router's job.
- Confusing a port number with an IP address.
- Believing HTTPS hides the destination domain completely: DNS is often plaintext and the TLS handshake exposes the server name.
- Treating UDP as a broken TCP rather than a deliberate latency trade-off.