Middleboxes: NATs

Introduction

Network Address Translation (NAT) was proposed as a short term solution to deal with the expected shortage of IPv4 addresses in the late 1980s to early 1990s. Combined with CIDR, NAT helped to significantly slow down the consumption of IPv4 addresses. A NAT is a middlebox that interconnects two networks that are using IPv4 addresses from different addressing spaces. Usually, one of these addressing spaces is the public Internet while the other is using a private IPv4 address. Unlike a router, when a NAT box forwards traffic, it modifies the IP addresses in the IP header, as will be described shortly.

Broadband Access Routers

A very common deployment of NAT is in broadband access routers as shown in the figure below. The broadband access router interconnects a home network, either WiFi or Ethernet-based, and the global Internet via one ISP.

A single IPv4 address is allocated to the broadband access router and network address translation allows all of the hosts attached to the home network to share a single public IPv4 address.