The external network

The features of an external or a public network are as follows:

• It provides global connectivity and uses routable IP addressing
• It is used by the virtual router to perform SNAT from the VM instances and provide external access to traffic originating from the VM and going to the Internet

SNAT refers to Source Network Address Translation. It allows traffic from a private network to go out to the Internet. OpenStack supports SNAT through its Neutron APIs for routers. More information can be found at http://en.wikipedia.org/wiki/Network_address_translation.

• It is used to provide a DNAT service for traffic from the Internet to reach a service running on the VM instance

While using VLANs, by tagging networks and combining multiple networks into one Network Interface Card (NIC), you can optionally leave the public network untagged for that NIC, to make the access to the OpenStack dashboard and the public OpenStack API endpoints simple.