Your GitHub Enterprise installation may be behind some combination of firewalls and VPNs at your company. If you would like to access GitHub Enterprise from outside those security layers, you will need to open a few ports.
Specifically, you may need to allow the following connections:
|22||SSH||Used for Git over SSH and admin shell access|
|80||HTTP||Only used when HTTPS/SSL is disabled|
|443||HTTPS||Only used when HTTPS/SSL is enabled|
|9418||Git protocol||Only used when private mode is disabled|
|13337||HTTP||Used for remote access to the Management Console|