Data Communication and Network Fundamentals
Layered models and encapsulation
You must be able to map the OSI seven layers onto the TCP/IP four-layer model and say which device works where: hubs/repeaters at L1, switches and bridges at L2, routers at L3, firewalls and load balancers at L4-L7. Encapsulation adds a header per layer and the PDU is renamed at each step (data → segment → packet → frame → bits). Know the header sizes: IPv4 base header 20 bytes, TCP base header 20 bytes, Ethernet frame 64-1518 bytes with a 1500-byte default MTU.
Transport and helper protocols
TCP is connection-oriented: three-way handshake (SYN, SYN-ACK, ACK), sequencing, acknowledgements, sliding window, four-way close. UDP is connectionless with no retransmission. Memorise common ports: 20/21 FTP, 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 67/68 DHCP, 69 TFTP, 80 HTTP, 110 POP3, 161/162 SNMP, 443 HTTPS. ARP resolves IPv4 to MAC (request broadcast, reply unicast, plus gratuitous ARP); ICMP carries echo request/reply and time-exceeded used by ping and tracert.
Traps that appear in questions
Half-duplex links use CSMA/CD; full-duplex links do not, so a duplex mismatch causes collisions and late collisions on the full-duplex side. Every switch port is its own collision domain; a broadcast domain ends at a router or a VLAN boundary. A MAC address is 48 bits written as 12 hex digits; the first byte's low bit distinguishes unicast from multicast, and the second-lowest bit marks locally administered addresses.
Rule of thumb
On one switch, the number of collision domains equals the number of active ports, and the number of broadcast domains equals the number of VLANs (plus one per routed interface). If a question asks how many broadcast domains exist, count subnets/VLANs, not devices.