Network Address Translation (NAT) is a process for modifying
the source or destination addresses in the headers of an IP packet while
the packet is in transit. In general, the sender and receiver
applications are not aware that the IP packets are being
manipulated.

NAT is often implemented by routers, and so we will refer to the host
performing NAT as a NAT router. However, in OpenStack
deployments it is typically Linux servers that implement the NAT
functionality, not hardware routers. These servers use the iptables
software package to implement the NAT functionality.

There are multiple variations of NAT, and here we describe three
kinds commonly found in OpenStack deployments.

SNAT

In Source Network Address Translation (SNAT), the NAT router
modifies the IP address of the sender in IP packets. SNAT is commonly
used to enable hosts with private addresses to communicate with
servers on the public Internet.

RFC 1918 reserves
the following three subnets as private addresses:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

These IP addresses are not publicly routable, meaning that a host on
the public Internet can not send an IP packet to any of these addresses.
Private IP addresses are widely used in both residential and corporate
environments.

Often, an application running on a host with a private IP address
will need to connect to a server on the public Internet. An example is a
user who wants to access a public website such as www.openstack.org. If
the IP packets reach the web server at www.openstack.org with a private
IP address as the source, then the web server cannot send packets back
to the sender.

SNAT solves this problem by modifying the source IP address to an IP
address that is routable on the public Internet. There are different
variations of SNAT; in the form that OpenStack deployments use, a NAT
router on the path between the sender and receiver replaces the packet’s
source IP address with the router’s public IP address. The router also
modifies the source TCP or UDP port to another value, and the router
maintains a record of the sender’s true IP address and port, as well as
the modified IP address and port.

When the router receives a packet with the matching IP address and
port, it translates these back to the private IP address and port, and
forwards the packet along.

Because the NAT router modifies ports as well as IP addresses, this
form of SNAT is sometimes referred to as Port Address
Translation (PAT). It is also sometimes referred to as NAT
overload.

OpenStack uses SNAT to enable applications running inside of
instances to connect out to the public Internet.

DNAT

In Destination Network Address Translation (DNAT), the NAT
router modifies the IP address of the destination in IP packet
headers.

OpenStack uses DNAT to route packets from instances to the OpenStack
metadata service. Applications running inside of instances access the
OpenStack metadata service by making HTTP GET requests to a web server
with IP address 169.254.169.254. In an OpenStack deployment, there is no
host with this IP address. Instead, OpenStack uses DNAT to change the
destination IP of these packets so they reach the network interface that
a metadata service is listening on.

One-to-one NAT

In one-to-one NAT, the NAT router maintains a one-to-one
mapping between private IP addresses and public IP addresses. OpenStack
uses one-to-one NAT to implement floating IP addresses.