What is an Ingress resource in Kubernetes?
It is a Kubernetes technique of exposing services via an individual IP address (page 135 of Kubernetes in Action by Luksa). In TCP/IP networking, the Ingress port allows inbound traffic to route somewhere. Kubernetes supports other IP address to service mapping methods (e.g., NodePort or LoadBalancer). NodePort operates on layer 4 of the OSI seven-layer model (according to this posting). The Ingress resource operates at layer 7 of the OSI model (page 135 of Kubernetes in Action by Luksa).
The ingress service supports networking rules based on the requestor's originating IP address (page 402 of Kubernetes in Action by Luksa).