Editing Network address translation (section)

=={{Anchor|NAT loopback}}NAT hairpinning==
'''NAT hairpinning''', also known as  '''NAT loopback''' or '''NAT reflection''',<ref>{{cite web|url=http://www.nycnetworkers.com/real-world/nat-reflectionnat-loopbacknat-hairpinning/ |title=What is NAT Reflection/NAT Loopback/NAT Hairpinning? |publisher=NYC Networkers |date=2014-11-09 |access-date=2017-04-27}}</ref> is a feature in many consumer routers<ref>{{cite web|url=http://opensimulator.org/wiki/NAT_Loopback_Routers |title=NAT Loopback Routers – OpenSim |publisher=[[OpenSimulator]] |date=2013-10-21 |access-date=2014-02-21 |format=[[MediaWiki]]}}</ref> where a machine on the [[LAN]] is able to access another machine on the LAN via the external IP address of the LAN/router (with port forwarding set up on the router to direct requests to the appropriate machine on the LAN). This notion is officially described in 2008, {{IETF RFC|5128}}.

The following describes an example network:
* Public address: {{IPaddr|203.0.113.1}}. This is the address of the [[Wide area network|WAN]] interface on the router.
* Internal address of router: {{IPaddr|192.168.1.1}}
* Address of the server: {{IPaddr|192.168.1.2}}
* Address of a local computer: {{IPaddr|192.168.1.100}}

If a packet is sent to {{IPaddr|203.0.113.1}} by a computer at {{IPaddr|192.168.1.100}}, the packet would normally be routed to the [[default gateway]] (the router){{efn|Unless an explicit route is set in the computer's [[routing]] tables.}}. A router with the NAT loopback feature detects that {{IPaddr|203.0.113.1}} is the address of its WAN interface, and treats the packet as if coming from that interface. It determines the destination for that packet, based on DNAT (port forwarding) rules for the destination. If the data were sent to port 80 and a DNAT rule exists for port 80 directed to {{IPaddr|192.168.1.2}}, then the host at that address receives the packet.

If no applicable DNAT rule is available, the router drops the packet. An [[ICMP Destination Unreachable]] reply may be sent. If any DNAT rules were present, address translation is still in effect; the router still rewrites the source IP address in the packet. The local computer ({{IPaddr|192.168.1.100}}) sends the packet as coming from {{IPaddr|192.168.1.100}}, but the server ({{IPaddr|192.168.1.2}}) receives it as coming from {{IPaddr|203.0.113.1}}. When the server replies, the process is identical to an external sender. Thus, two-way communication is possible between hosts inside the LAN network via the public IP address.