What is the cluster IP in Kubernetes?

Run this

$ kubectl cluster-info

It shows result like this where you can see the Kubernetes master IP

ClusterIP can mean 2 things: a type of service which is only accessible within a Kubernetes cluster, or the internal ("virtual") IP of components within a Kubernetes cluster. Assuming you're asking about finding the internal IP of a cluster, it can be accessed in 3 ways (using the simple-nginx example):

1. Via command line kubectl utility:

   $ kubectl describe service my-nginx
   Name:           my-nginx
   Namespace:      default
   Labels:         run=my-nginx
   Selector:       run=my-nginx
   Type:           LoadBalancer
   IP:         10.123.253.27
   LoadBalancer Ingress:   104.197.129.240
   Port:           <unnamed>   80/TCP
   NodePort:       <unnamed>   30723/TCP
   Endpoints:      10.120.0.6:80
   Session Affinity:   None
   No events.
   

2. Via the kubernetes API (here I've used kubectl proxy to route through localhost to my cluster):

   $ kubectl proxy &
   $ curl -G http://localhost:8001/api/v1/namespaces/default/services/my-nginx
   {
     "kind": "Service",
     "apiVersion": "v1",
     "metadata": <omitted>,
     "spec": {
       "ports": [
         {
           "protocol": "TCP",
           "port": 80,
           "targetPort": 80,
           "nodePort": 30723
         }
       ],
       "selector": {
         "run": "my-nginx"
       },
       "clusterIP": "10.123.253.27",
       "type": "LoadBalancer",
       "sessionAffinity": "None"
     },
     "status": {
       "loadBalancer": {
         "ingress": [
           {
             "ip": "104.197.129.240"
           }
         ]
       }
     }
   }
   

3. Via the $<NAME>_SERVICE_HOST environment variable within a Kubernetes container (in this example my-nginx-yczg9 is the name of a pod in the cluster):

   $ kubectl exec my-nginx-yczg9 -- sh -c 'echo $MY_NGINX_SERVICE_HOST'
   10.123.253.27
   

More details on service IPs can be found in the Services in Kubernetes documentation, and the previously mentioned simple-nginx example is a good example of exposing a service outside your cluster with the LoadBalancer service type.