Ephemeral IP not getting changed after stop and start

Question

As per the Google Cloud Documentation

Ephemeral external IP address are released from a resource if you delete the resource. For VM instances, if you stop the instance, the IP address is also released. Once you restart the instance, it is assigned a new ephemeral external IP address.

I just did try it with one of my GCloud compute and I see that the ephemeral IP did not change.

I used below command

gcloud compute instances stop test-compute-1 --zone='us-east4-a'

gcloud compute instances start test-compute-1 --zone='us-east4-a'

Is it a bug or feature?

It reassigns the ephemeral IP if it was associated with compute and available in IP pool.

Not sure what ephemeral means on GCloud though the definition is similar as per the documentation.

Note: Yes I did verify multiple times and IP assigned to my compute is marked as Ephemeral not Static.

Answer 1

I did some testing on my side, I created four VMs and stopped and started them as as fast as possible in order to randomize the order and these are the results I got.

v1

107.178.209.1xx

34.67.136.1xx

107.178.209.1xx

107.178.209.1xx

---

v2

35.222.128.2xx

107.178.209.1xx

25.222.109.1xx

34.67.136.1xx

---

v3

35.222.109.1xx

34.70.38.1xx

35.193.244.2xx

35.222.109.1xx

---

v4

35.193.244.2xx

35.222.128.2xx

35.222.128.2xx

34.70.38.1xx

---

you can assume that if the IPs have the same first 24 bits its the same IP even though I removed some bits.

After doing this test, it would lead me to conclude that the system tends to assign familiar IPs to VMs if It can. It would also seem that there is a limitation with the size of the ip pool. Hope this helps.