What are loopback and localhost?

Why do they exist, what are they used for, and how would I use them?

  • Isn't it two separate question in 1?
    – Anwar
    Apr 9, 2017 at 19:07

1 Answer 1


The loopback device is a special, virtual network interface that your computer uses to communicate with itself. It is used mainly for diagnostics and troubleshooting, and to connect to servers running on the local machine.

The Purpose of Loopback

When a network interface is disconnected--for example, when an Ethernet port is unplugged or Wi-Fi is turned off or not associated with an access point--no communication on that interface is possible, not even communication between your computer and itself. The loopback interface does not represent any actual hardware, but exists so applications running on your computer can always connect to servers on the same machine.

This is important for troubleshooting (it can be compared to looking in a mirror). The loopback device is sometimes explained as purely a diagnostic tool. But it is also helpful when a server offering a resource you need is running on your own machine.

For example, if you run a web server, you have all your web documents and could examine them file by file. You may be able to load the files in your browser too, though with server-side active content, it won't work the way it does when someone accesses it normally.

So if you want to experience the same site others do, the best course is usually to connect to your own server. The loopback interface facilitates that.

Addresses on Loopback

For IPv4, the loopback interface is assigned all the IPs in the address block. That is, through all represent your computer. For most purposes, though, it is only necessary to use one IP address, and that is This IP has the hostname of localhost mapped to it.

Thus, to log in as bob via SSH to the SSH server running on your own machine, you would use:

ssh bob@localhost

Like other network adapters, the loopback device shows up in the output of ifconfig. Its name is lo.

ek@Del:~$ ifconfig lo
lo        Link encap:Local Loopback  
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:50121 errors:0 dropped:0 overruns:0 frame:0
          TX packets:50121 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:4381349 (4.3 MB)  TX bytes:4381349 (4.3 MB)

An Example: CUPS

One common, production (i.e., not just diagnostic) use of localhost on Ubuntu is to perform advanced printer configuration. In a web browser, go to:


CUPS runs a web server on port 631, and this can be used to configure printing, regardless of what GUI you are running (or even if you are not running a GUI at all).

Screenshot showing CUPS in a web browser

If you try connecting to, this will work too. However, if you try to connect to, it will not. All the 127.*.*.* addresses identify your computer on the loopback interface, but a server program can decide to bind just to a specific IP address.

A Notable Difference from Windows

If you come from a Windows background, you might expect loopback to itself be a synonym of localhost (and thus to be able to ping loopback, connect to servers on loopback, and so forth). That behavior is peculiar to Windows.

Other Meanings of "Loopback"

The general concept of loopback is a mechanism through which a message or signal ends up (or loops) back to where it started.

So there are a few other ways loopback is use in Ubuntu that should not be confused with the loopback device in networking.

Loop Mounts

To mount a disk image in Ubuntu, you could run:

sudo mount -o loop image.iso /media/label

This is usually called a loop device (and not a loopback device), but the term loopback file interface is occasionally used.

This has nothing to do with the loopback device in networking.


Pulseaudio and other sound systems provide a mechanism to "connect" line-in to line-out, so that audio input is echoed back to your speakers/headphones. Pulseaudio's loopback module facilitates this.

Here, it is correct to use the term loopback, but like loop mounts, this also has nothing to do with the loopback device in networking. (And nothing to do with loop mounts, either.)

Further Reading

  • 8
    Why does map to through instead of through (Sorry if this is perceived as a highjack.)
    – Jellicle
    Jun 25, 2015 at 22:48
  • 7
    He mentioned only usable host addresses, and excluded network and broadcast addresses. You are correct in saying that it should be, but the first and last have other purposes. Jul 14, 2015 at 14:32
  • 2
    @Eliah, Must localhost be pointing to Or could it also point to another loopback address e.g.
    – Pacerier
    Jan 21, 2016 at 21:03
  • 2
    @GabrielSamfira, so why when I try to connect to those two IP addresses using SSH (e.g. by running ssh, it will say "Network is unreachable" instead of "Connection refused"? Aug 25, 2018 at 11:20
  • 6
    @MAChitgarha The first IP address in a subnet is the network identification address, and the last IP address is the broadcast address. Neither of these can be assigned to a host by default. The broadcast address is used to address every host in a network, and the network address is used to ID the network itself. Please see: en.wikipedia.org/wiki/IP_address Aug 25, 2018 at 14:47

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