Everywhere I looked I was unable to find the IPs to whitelist for a Service Hook that reaches back to our internal network from Azure Devops. I was able to get it to work by making the request and looking at the logs what IP was blocked. I then went to ARIN and looked up the IPs and found the blocks that Microsoft is using for this service.
As of 3/10/2020 -
Net Range 20.33.0.0 - 20.128.255.255 CIDR 20.33.0.0/16 20.34.0.0/15 20.36.0.0/14 20.40.0.0/13 20.48.0.0/12 20.64.0.0/10 20.128.0.0/16
Sorry for any inconvenience.
At present, there is no specified Azure DevOps Service Hooks IP address. You could try to add Azure DevOps IP addresses:
Allowed address lists and network connections
But, that is Azure Datacenter Ips, Azure Datacenter IPs are a superset of it, but if you want have a smaller IP list to white list, there is a suggestion ticket about that feature:
Webhooks (service hook) inbound IP whitelist
You could vote and add your comments for this feedback.
Hope this helps.