Networking Concepts: Models, Ports, and Addressing
Layered models
Know the OSI layers in order - Physical, Data Link, Network, Transport, Session, Presentation, Application - and map devices and protocol data units to them. Switches and MAC addresses live at Layer 2; routers and IP packets at Layer 3; TCP/UDP ports and segments at Layer 4. The TCP/IP model compresses this into link, internet, transport, and application, so a question may ask for the OSI equivalent of a TCP/IP layer.
Ports and protocols
Memorize service ports cold: SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, HTTP 80, HTTPS 443, LDAP 389 (636 for LDAPS), SMB 445, RDP 3389, SNMP 161 with traps on 162. DNS uses UDP for ordinary queries and TCP for large responses and zone transfers.
Addressing
IPv4 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16; 127.0.0.0/8 is loopback and 169.254.0.0/16 is link-local (APIPA). In any subnet the all-zeros host is the network address and the all-ones host is broadcast. IPv6 is 128 bits, has no broadcast (multicast replaces it), and uses fe80::/10 for link-local.
Traps
Questions often ask for usable hosts rather than total addresses, or present an OSI layer for a function that spans layers - TLS, for example, is usually described at the transport/session boundary. A multilayer switch routes at Layer 3, so "switch = Layer 2 only" is not always safe.
Rule of thumb
For IPv4, usable hosts = 2^(32 − prefix) − 2, except /31 point-to-point links (2 usable) and /32 host routes. Establish the block size first, then count network and broadcast as unusable.