You need to expose mongodb using Kubernetes Services. With the help of Services there is no need for an application to know the actual IP address of the Pod, you can use the service name to resolve mongodb.

Reference: https://kubernetes.io/docs/concepts/services-networking/service/

An example using mysql:

apiVersion: v1
kind: Service
metadata:
  labels:
    name: mysql
  name: mysql
spec:
  ports:
    - port: 3306
  selector:
    name: mysql
---
apiVersion: v1
kind: Pod
metadata:
  name: mysql
  labels:
    name: mysql
spec:
  containers:
      image: mysql
      name: mysql
      env:
        - name: MYSQL_ROOT_PASSWORD
          value: wppassword
      ports:
        - containerPort: 3306
          name: mysql

If there is application container, in the same namespace, trying to use the mysql container, it can directly use mysql:3306 to connect with out using the POD IP address. And mysql.namespace_name:3306 if the app is in a different namespace.