Update README.md: add IP addresses information.

README.md

@@ -52,6 +52,15 @@ Container Registry images uploaded to GitLab Registry Service doesn't have size
 
 For large size repository, consider to use [GitLab LFS](https://docs.gitlab.com/ee/workflow/lfs/manage_large_binaries_with_git_lfs.html).
 
+##  What are the IP addresses I should whitelist if I run services behind firewall?
+
+- GitLab instance is behind a NAT gatway. The static IP is **35.233.234.95**. If you use webhooks to other services that
+is behind a firewall, you need to whitelist its NAT address.
+
+- GitLab public service IP (code.stanford.edu) is **35.233.234.95**. If your network has outgoing firewall, you
+need to whitelist code.stanford.edu's public IP.
+
+
 ## How to generate and upload GitLab ssh-key?
 
 * [Generate ssh-key](http://doc.GitLab.com/ce/ssh/README.html)