certain ip addresses can be assigned freely on the local area network. since they are not unique, they are not routed on the internet. the addresses which can be freely assigned are called private ip addresses, and the ones which are unique are called public. to be routed, a private address needs to be translated into a public one. this process of translating between private and public addresses is called network address translation, or nat. routers and firewalls often also perform this task.