Loopback IP address

Often when you want to test a TCP/IP protocol suite on your computer's network interface card (NIC), you will run the following command from your computer's Command Prompt:

C:\>ping 127.0.0.1

The successful reply will look similar to the following:

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

Ping statistics for 127.0.0.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
Approximate round trip times in mili-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms 

The 127.0.0.1 address is often called a loopback IP address, which comes from the private IP address range 127.0.0.0 - 127.255.255.255, which is reserved for loopback purposes and diagnostic functions. As such, it is part of the loopback adapter, which is a virtual network card that is isolated from an external network.