2021 update
142.250.82.77
142.250.82.113
142.250.82.28
142.250.82.126
2020 update
142.250.82.42
142.250.82.71
142.250.82.78
209.85.144.127
74.125.250.71
Suspected additional IPs
And I observed the user agent “Google Hangouts” trying these IPs, but by the FQDN mtalk.google.com (whose resolution must by very dynamic) rather than raw IP:
142.250.31.188
172.253.122.188
172.253.63.188
Older IPs
173.194.207.127
209.85.232.127
64.233.177.127
64.233.177.103
74.125.196.127
They all have been used by Google’s Hangouts Meet based on my observation.
If you have an environment which uses proxy authentication, the above IPs do not play well with that. So you’ll need to disable proxy authentication for them for Hangouts Meet to work.
Otherwise you can do the initial connect but will be dropped after about 45 seconds.
Finally, although you can look up each individually and learn of its association to Google, Google’s own documentation is devoid of any references to them. That is unfortunate.
So the IPs in actual use is probably much larger, but these are what I’ve observed over the course of a few days of testing.
What I’ve done
Google is very hard to reach. They only provide indirect means for regular users. So not knowing any insiders I submitted feedback at https://meet.google.com/ which is what they suggest. I provided a detailed description. I doubt they will do anything with my feedback. We’ll see. Update. Correct. Months have passed and they never bothered to get back to me.
Conclusion
Google has an undocumented dependency on a whole set of IPs for hangout Meets to actually work through a proxy which requires authentication. Contacting Google for more information is probably impossible, but I will try.