Networking interview questions

Model answer

The browser resolves the domain to an IP via DNS, opens a TCP connection to that IP on port 443, performs a TLS handshake to establish encryption and verify the certificate, then sends an HTTP request. The server responds, possibly through a load balancer and CDN, and the browser renders the result. Each step is a place things can break.

Model answer

TCP is connection-oriented and reliable: it establishes a connection, guarantees ordered delivery, and retransmits lost packets, which suits web traffic and databases. UDP is connectionless and best-effort with no ordering or retransmission, which suits low-latency, loss-tolerant traffic like DNS, video, and gaming.

Model answer

CIDR notation like 10.0.0.0/16 marks how many leading bits are the network portion; the rest address hosts. A /16 has 65,536 addresses, a /24 has 256. Subnetting splits a block into smaller networks for isolation and routing, for example public and private subnets within a VPC.

Model answer

A forward proxy sits in front of clients and forwards their requests outward, often for filtering or caching on the client side. A reverse proxy sits in front of servers and accepts requests on their behalf, providing load balancing, TLS termination, and caching. NGINX as a reverse proxy is the classic example.

Model answer

A maps a name to an IPv4 address and AAAA to IPv6. CNAME aliases one name to another. MX directs mail. TXT holds arbitrary text, used for SPF, DKIM, and domain verification. NS delegates a zone. TTL controls caching, which is why DNS changes can take time to propagate.

Model answer

Walk the stack methodically: confirm the target resolves with DNS, check the route exists, verify the port is listening with ss, check firewall or security group rules, and test connectivity with curl or telnet. Tools like tcpdump and mtr show exactly where packets stop. Isolate the layer before guessing.

Model answer

Layer 4 load balancing operates on TCP/UDP and routes by IP and port without inspecting content, so it is fast and protocol-agnostic. Layer 7 understands HTTP and can route by host, path, header, or cookie, enabling features like path-based routing and TLS termination at the cost of more processing.

Model answer

The browser resolves the hostname through DNS, opens a TCP connection and a TLS handshake for HTTPS, then sends an HTTP request. The server, often behind a load balancer and reverse proxy, routes the request to an app instance, which responds with HTML. The browser then fetches linked assets and renders the page. Knowing each hop helps you reason about where latency or failures come from.