ClusterIP

ClusterIP is the default Service type in Kubernetes, but it is not meant for external access. 

Comparison with other mechanisms

Method	Layer	External Access?	Pros	Cons	Typical Use Case
ClusterIP	L4	No	Simple, lightweight, internal only	Not accessible from outside the cluster	Backend services, microservices talking to each other
NodePort	L4	Yes	Easy to setup, no extra resources	Uses high ports (30000-32767), not ideal for production	Testing, development
LoadBalancer	L4	Yes	Cloud provider managed LB, simple	Expensive (one LB per service), limited routing	Simple public services
Ingress	L7	Yes	Single entry point, smart routing, TLS	Requires Ingress Controller	Most production web apps

So what is ClusterIP exactly ?

• Purpose: Allows pods to communicate with each other inside the cluster.
• It gets a virtual IP (ClusterIP) that is only reachable from within the Kubernetes cluster.
• Other services/pods can reach it using the service name (e.g., http://my-api-service:80).
• It does not open any ports to the outside world.

Example:

apiVersion: v1
kind: Service
metadata:
  name: backend
spec:
  type: ClusterIP     # This is default, even if you don't specify
  selector:
    app: backend
  ports:
  - port: 80
    targetPort: 8080

When to Use What

Need	Recommended Approach
Internal communication only	ClusterIP
Quick external access (dev/test)	NodePort
Simple external access on cloud	LoadBalancer
Multiple apps, domains, paths, TLS	Ingress (best choice)
Advanced routing, traffic management	Ingress or Gateway API