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127.0.0.0/8 127.0.1.1 127.0.0.1 – Ubuntu / Debian

Some time ago I wrote this article “what does 127.0.1.1 mean ?” and I explained why some applications like GNOME expects that the hostname to be resolved to an ip address with a canonical fully qualified domain name – FQDN.

Another interesting thing that I have not said about it is that the range 127.0.0.0/8 (IANA – Loopback) is entirely reserved for Loopback (RFC 5735).

In other words anything that you ping within that range (127.0.0.0 – 127.255.255.255) will reply back to the localhost (loopback).

$ ping 127.1.13.1
PING 127.1.13.1 (127.1.13.1) 56(84) bytes of data.
64 bytes from 127.1.13.1: icmp_req=1 ttl=64 time=0.072 ms

One piece of advice is to not change the defaults in Ubuntu / Debian systems otherwise you can expect weird errors from some applications. Also do not configure anything to listen on port 127.0.1.1. If you want to know what is currently running on the loopback range type the following

$ sudo netstat -tunelp | grep 127.0.[0-255].[0-255]
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      117        11880       1099/mysqld
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      0          13907       2061/dnsmasq
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      0          11116       1019/cupsd
tcp        0      0 127.0.0.1:9050          0.0.0.0:*               LISTEN      0          10944       1393/tor
udp        0      0 127.0.0.1:53            0.0.0.0:*                           0          13906       2061/dnsmasq
udp        0      0 127.0.0.1:715           0.0.0.0:*                           0          9455        963/rpc.statd
udp        0      0 127.0.0.1:34242         0.0.0.0:*                           1000       18458       2410/skype

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