Network Fundamentals
The Network Fundamentals domain carries 20% of the CCNA 200-301 exam and tests how data is encapsulated, addressed, and carried across LAN and WAN topologies. Questions often ask you to read a topology description, choose the right cable or device, or calculate subnet values - so precision with masks and PDU names matters more than memorizing marketing terms.
Encapsulation and the TCP/IP model
The OSI model's seven layers map to the four-layer TCP/IP model: application, transport, internet, and network access. Data, segments, packets, and frames are the PDU names at each step. TCP provides sequenced, acknowledged, connection-oriented delivery; UDP is connectionless with best-effort delivery and is preferred for voice, DNS, and DHCP. Recognize well-known ports: 22 SSH, 23 Telnet, 25 SMTP, 53 DNS, 67/68 DHCP, 80 HTTP, and 443 HTTPS.
IPv4 addressing and subnetting
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. A prefix length defines the mask: /24 = 255.255.255.0, /26 = 255.255.255.192, /27 = 255.255.255.224. The block size is 256 minus the interesting octet, which gives the subnet increment. Each subnet has a network address and a broadcast address, both unusable, with usable hosts between them. IPv6 uses /64 for most LANs and has no broadcast; link-local addresses start with fe80::/10.
Media, topologies, and virtualization
Copper Ethernet categories differ in supported speed and distance; fiber supports longer runs and immunity to EMI. PoE standards 802.3af (15.4 W), 802.3at (30 W), and 802.3bt (up to 90 W) power APs and phones. Two-tier collapsed core, three-tier, spine-leaf, SOHO, and WAN designs each place core, distribution, and access roles differently. Hypervisors and virtual switches extend Layer 2 domains, so VLAN and trunk design still governs traffic.
Worked example
A host is 172.16.5.130/26. The /26 mask is 255.255.255.192 with block size 64, so subnets begin at .0, .64, .128, and .192. The host falls in 172.16.5.128/26: network 172.16.5.128, broadcast 172.16.5.191, usable range 172.16.5.129–172.16.5.190.
Traps
Using 255.255.255.224 for /26 instead of .192; treating the broadcast as usable; confusing TCP's three-way handshake with UDP; and assuming all private ranges are /24.