Ports Aren't Numbers, They're Doors: A Mental Model for SREs
An interview question about ports is not a trivia quiz. It's a test of your mental map of a distributed system. A junior engineer memorizes numbers. A senior engineer understands what conversations happen behind each door. Let's build the right mental model.
District 1: Public Infrastructure
These are the fundamental utilities of your digital city. If these are blocked, nothing else matters.
22 → SSHThe secure maintenance tunnels. The primary way engineers get privileged access to fix things directly.80 → HTTPThe old, unsecured public entrance. Mostly a redirect to the secure lobby now.443 → HTTPSThe main, secure public lobby. All user traffic, all API calls, enter here. The most important door to keep open.53 → DNSThe city's address book. How names ("google.com") are translated into physical locations (IP addresses).25/587 → SMTPThe city's post office. How all emails and alerts are sent out.123 → NTPThe city's clock tower. Ensures every system in the city agrees on the exact time, which is critical for logging and coordination.
District 2: The Observability Department
This is the city's nervous system and census bureau. These doors allow you to understand the health and state of the city.
9090 → PrometheusThe census bureau's main office. Where all vital statistics (metrics) about the city are collected and queried.9100 → Node ExporterThe census worker on every block (server), reporting local statistics back to the main office.3000 → GrafanaThe mayor's situation room. Visualizes the data from the census bureau into understandable dashboards.9093 → AlertmanagerThe emergency broadcast system. Takes alerts from Prometheus and routes them to the right responders.9200/9300 → ElasticsearchThe city's public library and archive. A searchable database for all logs and documents. 9200 is for the public API, 9300 for internal cluster communication.5601 → KibanaThe librarian's desk. The user interface for searching the Elasticsearch archives.8080 → Jenkins UIThe construction site foreman's office. The main UI for the CI/CD system.
District 3: The Government & Control Plane
This is city hall. These are the highly privileged doors where the laws of the system are written and enforced. Securing these is paramount.
6443 → Kubernetes API ServerThe Mayor's Office. This is the central control plane for the entire Kubernetes cluster. All `kubectl` commands go here.10250 → Kubelet APIThe local precinct captain on every block (node). Takes orders from the API server and manages containers locally.10257/10259 → Controller Mgr / SchedulerThe city planning department. Decides which new buildings (pods) get built and where.2375/2376 → Docker APIThe local building inspector's office. The API for controlling the container runtime directly on a node. 2376 is the secure version.30000-32767 → K8s NodePortPublicly accessible PO Boxes. A range of ports Kubernetes can assign to services to expose them to the outside world.
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