Extension resource groups of provider network services

At a network manager of an extension resource group of a provider network, a message comprising a command to launch a compute instance is received at an address which is part of a first network configured at a premise external to the provider network. The extension resource group includes a first host at the external premise. Within a second network configured at the external premise, the first host is assigned an address within a second address range. Addresses within the second range are also assigned to hosts within the provider network. The command is transmitted to the first host, and a compute instance is instantiated.