A technology is described for a managed NAT (Network Address Translation) system. An example method of the NAT system may include receiving a request to launch a NAT system for a computing network that includes a number of computing devices. The NAT system may be used to allocate network addresses and ports for computing instances executing on one or more host computing devices according to a NAT policy. The NAT policy may include specifications that may be obtained and used to determine allocation of network addresses to the computing instances. The NAT system may then be associated with the computing devices included in the computing network and the NAT policy may be applied among the computing devices.