Network Fundamentals, Architecture, and Topologies
This domain tests how data moves from an application to the wire and how topology choices affect failure domains, latency, and policy placement. Expect encapsulation, addressing, subnetting, VLAN boundaries, and architecture patterns such as three-tier, collapsed core, spine-leaf, and hub-and-spoke.
Core rules
- OSI layers map to TCP/IP: application, transport, internet, and link. Each layer adds a header; a frame carries a packet, which carries a segment or datagram.
- Switches forward by destination MAC within a VLAN; routers forward by destination IP between subnets. A VLAN is a broadcast domain, not a security boundary by itself.
- IPv4 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. NAT translates private to public addresses but does not replace routing.
- Spine-leaf connects every leaf to every spine, giving predictable east-west latency. Three-tier uses access, distribution, and core.
- CIDR determines subnet size: usable hosts = 2^h - 2, where h is the number of host bits.
Worked example
For 192.168.10.0/26, the mask is 255.255.255.192 and the block size is 64. Subnets are .0, .64, .128, and .192. The first subnet has network .0, usable .1-.62, and broadcast .63. A host at .70 belongs to .64/26, usable .65-.126, broadcast .127. Reasoning: 32-26=6 host bits, 2^6=64 addresses, then subtract network and broadcast.
Traps
Do not call a /26 a /24. Do not assign network or broadcast addresses to hosts. A collision domain ends at a switch port; a broadcast domain ends at a router or VLAN boundary. APIPA 169.254.0.0/16 means DHCP failed, not a valid routed address. Hub-and-spoke centralizes policy but adds path delay.