Network Fundamentals
Models and encapsulation
Move fluently between the OSI seven-layer model and the four-layer TCP/IP model. The transport layer adds TCP or UDP headers (segments/datagrams), the internet layer adds IP headers (packets), and the data link layer wraps everything in an Ethernet frame with MAC addresses and an FCS trailer. Traps: questions asking which layer a device works at (hub/repeater = L1, switch/bridge = L2, router = L3), and the direction of encapsulation (top-down on send, bottom-up on receive).
Addressing
IPv4 essentials: private ranges 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; loopback 127.0.0.0/8; APIPA 169.254.0.0/16. IPv6 essentials: global unicast 2000::/3, link-local FE80::/10, unique local FC00::/7, multicast FF00::/8, loopback ::1. IPv6 has no broadcast, so any answer offering an IPv6 broadcast address is wrong by definition. SLAAC, EUI-64, and anycast are common distractors.
Switching and transport
Switches learn source MACs and flood unknown unicast, broadcast, and multicast frames. Count broadcast domains per VLAN or routed interface, not per port. Know the three-way handshake and well-known ports: 20/21 FTP, 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 67/68 DHCP, 69 TFTP, 80 HTTP, 443 HTTPS.
Rule of thumb
For any prefix, host bits n = 32 − prefix, addresses per subnet = 2^n, usable hosts = 2^n − 2. A /26 leaves 6 host bits: 64 addresses, 62 usable, boundaries at .0, .64, .128, .192 (256 − 64 = 64).