Network Fundamentals
Network fundamentals establish the vocabulary and models used throughout the CCNA 200-301 exam. This domain covers the roles of network devices, the OSI and TCP/IP models, physical media, and both IPv4 and IPv6 addressing. Mastering subnetting and address types here makes every later topic easier.
OSI and TCP/IP Models
The OSI model divides communication into seven layers, from Physical up to Application, giving each function a clear home. Layer 1 moves bits, Layer 2 frames data with MAC addresses, Layer 3 routes packets with IP addresses, and Layer 4 provides end-to-end transport with TCP or UDP. The TCP/IP model condenses these into four layers but maps closely to OSI. Understanding encapsulation, where each layer adds a header, is central to troubleshooting.
Network Components and Media
Routers connect and route between different networks at Layer 3, while switches forward frames within a LAN at Layer 2. Endpoints, access points, firewalls, and controllers each play distinct roles. Copper twisted-pair cabling (straight-through and crossover) suits short runs, while fiber supports long distances and high speeds. Choosing the correct cable and interface type prevents physical-layer faults.
IPv4 Addressing and Subnetting
An IPv4 address is 32 bits split into network and host portions by a subnet mask. Subnetting borrows host bits to create additional networks, and the block size (256 minus the mask octet value) reveals subnet boundaries. The network address has all host bits zero and the broadcast has them all one. VLSM lets you size subnets efficiently to match host counts.
IPv6 Addressing
IPv6 uses 128-bit addresses written as eight hextets, with rules to compress leading zeros and one run of zeros using a double colon. Global unicast addresses are routable, link-local (FE80::/10) addresses serve on-link functions, and unique local addresses are private. SLAAC lets hosts self-configure from a Router Advertisement prefix. Multicast replaces IPv4 broadcast for one-to-many delivery.
Transport Layer Protocols
TCP is connection-oriented, using a three-way handshake, sequencing, acknowledgements, and flow control to guarantee reliable delivery. UDP is connectionless and lightweight, favoring speed for voice, video, and DNS. Port numbers identify applications, with well-known ports below 1024. Recognizing when reliability or low latency matters guides protocol selection.