Network Fundamentals
Network Fundamentals
Network Fundamentals carries a large share of the LCPE blueprint and tests whether you can explain how data moves from an application to a wire and back: the OSI and TCP/IP models, Ethernet media and duplex, IPv4/IPv6 addressing and subnetting, TCP versus UDP behavior, wireless principles, and virtualization concepts.
Core rules
- Encapsulation order: data → segment (TCP/UDP, port) → packet (IP, address) → frame (MAC) → bits. Each layer reads only its own header.
- A switch learns MAC addresses from source frames and floods unknown, broadcast, and multicast frames. A router drops the incoming frame and rewrites Layer 2 at every hop.
- TCP is connection-oriented: three-way handshake (SYN, SYN-ACK, ACK), sequence numbers, windowing, retransmission. UDP is connectionless and used by DNS (53), DHCP (67/68), TFTP (69), SNMP (161/162).
- IPv4 private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. IPv6 uses 128-bit addresses, /64 on links, and NDP instead of ARP.
- Speed or duplex mismatch produces late collisions and CRC errors, so autonegotiation should match on both ends.
Worked example
Subnet 192.168.10.0/26. The mask 255.255.255.192 gives a block size of 256 − 192 = 64, so subnets are .0, .64, .128, .192. For .64: network 192.168.10.64, first host .65, last host .126, broadcast .127. The next subnet starts at .128, one above the broadcast - the fastest sanity check.
Traps
Assuming 192.168.10.64/26 broadcasts at .128; confusing collision domains (per switch port) with broadcast domains (per VLAN); forgetting a /31 has two usable addresses and no broadcast; and treating the default gateway as optional for inter-VLAN traffic.