natting

Natting, or Network Address Translation (NAT), is a technique used by routers and firewalls to translate between private IP addresses used inside a local network and public IP addresses used on external networks. NAT enables multiple devices to share a single public IP address and provides a layer of isolation between internal hosts and the Internet. It can also implement policy controls, logging, and access restrictions.

There are several NAT varieties. Static NAT creates a fixed one‑to‑one mapping between a private address and a public address. Dynamic NAT uses a pool of public addresses and assigns them to internal hosts as needed. Port Address Translation (PAT), also known as NAT overload, maps many private addresses to a single public address by multiplexing connections through different source ports. Some deployments combine these modes.

Operation: A NAT device keeps a translation table recording each internal source IP/port and its corresponding external public IP/port. For outbound traffic, it rewrites the source address and port in the IP header and updates the table. For return traffic, it uses the translation to route packets to the correct internal host. NAT is typically stateful, remembering active translations until timeout.

Effects and considerations: NAT conserves public addresses and can improve privacy by obscuring internal addressing. It can hinder end-to-end connectivity and complicate protocols that embed address information or require direct inbound connections, such as some peer-to-peer applications, VoIP, and FTP in active mode. Protocols may require care or helper mechanisms, like NAT traversal techniques or application-layer gateways.

Context: With the adoption of IPv6, NAT is less essential for address conservation, and some networks run with globally routable addresses. However, NAT remains widespread in IPv4 deployments due to address scarcity and security considerations, and is often configured on home and enterprise routers to manage traffic and access.