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For many years I thought of a subnet as a single contiguous range of ip addresses expressed using CIDR notation.

However on GCP a subnet can have a primary ip address range from which VMs get assigned an ip address a bunch of and secondary ip ranges that are used as ip aliases. For example GKE use one range for the kuberenetes pod network and another one for the kuberenetes service network.

Questions:

  • In TCP/IP can a subnet have more than one CIDR
  • Is the definition of subnet with mutiple CIDR ranges in GCP unique to GCP or a common across others products and services?
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For IPv4, running multiple subnets in the same segment isn't obvious but not uncommon. Sometimes it's used in a DMZ scenario to "hard" separate purely internal and external/mixed traffic. For instance, you could bind a database service to an internal subnet address only, so it can never be reached from the outside.

For IPv6, multiple address scopes are the rule. Commonly, you'd have a link-local, a unique-local, and a public address on each interface.

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Interfaces can have multiple IP addresses. They can be in different subnets.

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