Time for another question from the mailbag:

Why is the Loopback address always in the range of 127.0.0.0 to 127.255.255.255 ?

This range of IP addresses is reserved by the Internet Assigned Names Authority (IANA)  for use as the Internet host loopback address.  All Internet connected hosts (hosts running Internet Protocol) can use this adress for local loopback processing.  This address range is typically used for special purposes such as testing the network interfaces and the network software.

A ping sent to any address should always work if at least one interface is enabled. Here is an example of what happens on a Windows XP host when you ping an address in the 127.0.0.0 – 127.255.255.255 range:

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\inetdaemon>ping 127.127.127.127

Pinging 127.127.127.127 with 32 bytes of data:

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time=-63ms TTL=128
Reply from 127.0.0.1: bytes=32 time=-63ms TTL=128
Reply from 127.0.0.1: bytes=32 time=63ms TTL=128

Ping statistics for 127.127.127.127:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = -63ms, Average = 1073741808ms

The current list of reserved blocks of IP address space and their designated use are listed in RFC 3330 and RFC 1918.

Final Note:

Cisco routers have a virtual loopback interface and this should not be confused with the loopback address, even though the loopback address can be used on the Cisco loopback interface.

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