Editing Localhost (section)

==Packet processing==
The processing of any packet sent to a loopback address, is implemented in the [[link layer]] of the [[TCP/IP stack]]. Such packets are never passed to any [[network interface controller]] (NIC) or hardware device driver and must not appear outside of a computing system, or be routed by any router. This permits software testing and local services, even in the absence of any hardware network interfaces.

Looped-back packets are distinguished from any other packets traversing the TCP/IP stack only by the special IP address they were addressed to. Thus, the services that ultimately receive them respond according to the specified destination. For example, an HTTP service could route packets addressed to {{IPaddr|127.0.0.99:80}} and {{IPaddr|127.0.0.100:80}} to different Web servers, or to a single server that returns different web pages. To simplify such testing, the hosts file may be configured to provide appropriate names for each address.

Packets received on a non-loopback interface with a loopback source or destination address must be dropped. Such packets are sometimes referred to as [[Martian packet]]s.<ref>{{cite web|author=Raymond, Eric S.|title=The Jargon File|url=http://www.catb.org/~esr/jargon/html/M/martian.html}}</ref> As with any other bogus packets, they may be malicious and any problems they might cause can be avoided by applying [[bogon filtering]].