Networks and the Internet
Networks and the Internet tests how data moves between devices: layered protocols, addressing, routing, and common application services. You should be able to trace a request, identify a protocol’s layer, and perform basic subnet calculations.
Layering and protocols
The TCP/IP model has application, transport, internet, and link layers. The OSI reference model has seven layers: physical, data link, network, transport, session, presentation, application. IP provides logical addressing and best-effort packet delivery. TCP adds connection setup, sequencing, acknowledgements, and retransmission; UDP is connectionless with lower overhead. DNS resolves names to IP addresses. HTTP is an application protocol; HTTPS adds TLS encryption. Common ports include 80 for HTTP, 443 for HTTPS, 53 for DNS, and 25 for SMTP.
Addressing and routing
IPv4 addresses are 32 bits; IPv6 addresses are 128 bits. A subnet mask separates network and host bits. Routers forward packets between networks using routing tables; switches forward frames within a LAN using MAC addresses. Private IPv4 ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
Worked example
For 192.168.10.0/26, the mask has 26 network bits and 6 host bits. Host addresses per subnet = 2^6 − 2 = 62, because the all-zeros network address and all-ones broadcast address are reserved. The subnet increment is 64, so valid subnets include .0, .64, .128, and .192.
Traps
Do not place IP at the transport layer or TCP at the application layer. A hub broadcasts; a switch uses MAC addresses; a router uses IP addresses. In subnetting, subtract 2 for usable hosts unless the question asks for total addresses. DNS does not route packets; it resolves names.