NATs

NAT, or Network Address Translation, is a method used in IP networks to modify IP address information in packet headers as they traverse a network device such as a router or firewall. Its primary purpose is to enable multiple devices on a private network to share a single public IP address when communicating with the Internet, helping to mitigate IPv4 address exhaustion.

There are several forms of NAT. Static NAT provides a one-to-one mapping between a private address and a public address. Dynamic NAT uses a pool of public addresses and maps private addresses to any available public address from that pool. Port Address Translation (PAT), also known as NAT overload, maps many private addresses to a single public address by using different port numbers to distinguish connections.

How NAT works: a NAT device maintains a translation table. Outbound traffic from a private host has its source address and port replaced with the public address and a chosen port; inbound traffic is looked up in the table to be forwarded to the correct private internal host. Entries time out after a period of inactivity.

Private address spaces defined by RFC 1918 (such as 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16) are not routable on the public Internet and require translation to reach external networks. While NAT is widely used in IPv4 networks, IPv6 reduces the need for NAT, though some forms of translation such as NAT64 exist for IPv6–IPv4 interoperation.

Advantages of NAT include address conservation and an additional layer of network isolation. Limitations include disruption of end-to-end connectivity for certain applications and protocols, complications for inbound connections, and potential challenges for VPNs and some peer-to-peer services.