Networks and the Internet
Networks and the Internet
Items here test the path a message takes from one host to another: protocol layers, IP addressing and subnetting, naming and configuration services, and the security controls that protect traffic. You should be able to read a CIDR block, choose between TCP and UDP, and explain what a router does that a switch does not.
Core rules
- Layering: application (HTTP, DNS, SMTP), transport (TCP, UDP), internet (IP, ICMP), link (Ethernet, Wi-Fi). Each layer adds or reads its own header.
- TCP vs UDP: TCP is connection-oriented, ordered, and retransmits lost segments; UDP is connectionless with lower overhead, used for DNS queries, streaming, and VoIP.
- Addressing: IPv4 is 32 bits written as four octets; a /24 mask leaves 8 host bits. Private ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
- Devices: switches forward frames within a LAN by MAC address; routers forward packets between networks by IP address; firewalls filter by rule.
Worked example
Split 192.168.10.0/24 into four equal subnets. Borrow 2 host bits: 2^2 = 4 subnets, new prefix /26. Each subnet has 2^(32-26) = 64 addresses; 62 are usable after the network and broadcast addresses. The subnets are .0-.63, .64-.127, .128-.191, and .192-.255. The first usable host in the second subnet is 192.168.10.65, and its broadcast is 192.168.10.127.
Traps
Counting the network and broadcast addresses as usable hosts; confusing a router with a switch; assuming UDP is always unreliable at the application layer rather than simply unacknowledged; and forgetting that DNS resolves names while DHCP assigns addresses.