Networking Fundamentals

A 32-bit number that divides an IP address into network and host portions, determining which devices are on the same local network.

"With a subnet mask of 255.255.255.0, all devices sharing the same first three octets are on the same subnet and can communicate directly."

Classless Inter-Domain Routing notation that expresses an IP address and its subnet mask as a single string, e.g. 192.168.1.0/24, where /24 means the first 24 bits are the network prefix.

"We allocated the 10.0.1.0/24 block to the app subnet, giving us 254 usable host addresses for that tier."

The router address that a device uses to forward packets destined for networks outside its own subnet. Without it, a host can only communicate locally.

"After setting the default gateway to 192.168.1.1, the server could reach external APIs — before it could only talk to hosts on its local subnet."

A distributed hierarchical system that translates human-readable domain names (e.g. api.example.com) into IP addresses that routers can use to deliver traffic.

"When the DNS TTL expired after our migration, some users were still hitting the old server because their resolver had cached the previous A record."

Dynamic Host Configuration Protocol — a network service that automatically assigns IP addresses, subnet masks, default gateways, and DNS server addresses to devices when they join a network.

"Because DHCP assigns addresses dynamically, we set a static reservation for the print server so its IP never changes between reboots."

A technique where a router maps multiple private IP addresses to a single public IP address, allowing many devices to share one internet-facing address.

"All 200 office workstations share one public IP thanks to NAT — the router tracks outbound connections and routes returning packets to the correct internal host."

The three-step process (SYN → SYN-ACK → ACK) by which a client and server establish a reliable TCP connection before data is exchanged.

"High TCP handshake latency in our traces pointed to packet loss — each dropped SYN caused a 1-second retransmit delay before the connection opened."

A connectionless transport protocol that sends packets without establishing a connection or guaranteeing delivery, ordering, or error correction. Faster than TCP but unreliable.

"We chose UDP for the live video stream — a dropped frame is less disruptive than the retransmit delay TCP would introduce."

The time delay between a request being sent and the first byte of the response being received, typically measured in milliseconds. Affected by physical distance, routing hops, and processing time.

"Cross-region database calls added 120ms of latency to every API request — moving the read replica closer to the app servers cut it to 4ms."

Throughput is the actual rate of successful data transfer measured in practice; bandwidth is the maximum theoretical capacity of a link. High bandwidth does not guarantee high throughput if latency or packet loss is significant.

"The 1 Gbps link had plenty of bandwidth, but throughput was only 200 Mbps — TCP window scaling wasn't enabled, so the high latency link was under-utilised."

An encrypted tunnel over a public network that allows remote users or sites to communicate as if they were on the same private network.

"All developer access to production systems requires connecting via VPN — the firewall blocks direct SSH from public IP ranges."

A network security device or software that monitors and controls incoming and outgoing traffic based on a set of rules, blocking unauthorised connections.

"The firewall's default-deny rule blocked the new microservice's outbound calls to the payment API until we added an explicit allow rule for port 443."