Self-service project networks, when attached to a Neutron router, often utilize the router as their default gateway. By default, when a router receives traffic from an instance and routes it upstream, the router performs a port address translation and modifies the source address of the packet to appear as its own external interface address. When the translation occurs, the ephemeral source port is mapped to the original client address in a table that is referred to when the response packet is received. This ensures that the packet can be routed upstream and returned to the router, where the packet is modified and returned to the instance that initiated the connection. Neutron refers to this type of behavior as source NAT.

When users require direct inbound access to instances, a floating IP address can be utilized. A floating IP address in OpenStack is a one-to-one static NAT that maps an external address from an external network to an internal address in a project network. This method of NAT allows instances to be accessible from remote networks such as the internet. Floating IP addresses are configured on the external interface of the router that serves as the gateway for the instance, which is then responsible for modifying both the source and destination address of packets depending on their direction.