Original subtitles

Another special type of address is the Local Loopback Address

which is very useful for testing, a device can send a message

to itself and this helps verify or make sure that

the TCP/IP stack is installed correctly on that machine

A typical Loopback IP Address is 127.0.0.1 but remember please

that anything in the 127 range is deemed a Loopback Address

This is often seen as one of the big mistakes by the designers

of IPv4, a 127 address is a Class A Address

Class A Addresses have 16 million odd host addresses

and by chossing this address range for the Loopback Address

we lost the use of potentially 16 million IP Addresses

Host Addresses cannot be allocated as Public IP Addresses

on the Internet thus making the IPv4 Address space smaller than

it may have been, in IPv6 a different addresses used for the

loopback ::1 and they made sure that they didn't make

the same mistake again

So here's an example, I can type ipconfig on my PC

and this will give me the IP Address of my machine

In this example it's 10.0.0.6 that's the Local IPv4 Address

on my PC, so I could ping 10.0.0.6

and as you can see the ping succeeds

I can also however ping my Loopback Address

So I could ping 127.0.0.1

and notice the ping also succeeds

but don't forget anything in this ranges supported

so 127.127.127.127 is also a Loopback Address

What about 127.1.2.3?

it also works, so anything in the range 127.x.x.x

is also a Loopback Address and anyone of those addresses

could be used to test the TCP/IP Stack

Now this machine has both IPv4 and IPv6 installed

So here's my PC's IPv6 Address

So I could ping 2001:20::2 which is my Local PCs IP Address

but I could also ping ::1 which is my IPv6 Loopback Address

Now this video doesn't discuss IPv6 in any detail

But it's worth fighting out that we have both an IPv4 Loopback

such as 127.0.0.1 as well as an IPv6 Loopback such as ::1

IPv6 Addresses are not wasted or to be more politically correct

IPv6 Addresses are optimized more than they are in IPv4

Please also note that routers and switches

also have Loopback Addresses and that's not the same concept

as the Loopback Address that we're discussing here

So be aware that in the field or in the real world

Engineers will often refer to a routers Loopback Address

and that's not the same as what we're discussing here

You've got the Local Loopback which is the Class A Address

127.0.0.1 but routers and switches can also be configured

with valid IP Addresses on what's called a Loopback interface

So a router or switch may have a Loopback Interface such as

Interface Loopback 0, configured with an IP Address

of 10.1.1.1/32, so in other words we have a Class A Address

10.1.1.1 with a /32 mask configured on a Loopback Interface

but Network Engineers may refer to the router's Loopback

IP Address and they are referring to this IP Address

and not 127.0.0.1, I will explain as I've mentioned in a moment

what a Network mask does but what I'd like to point out here

is that routers and switches have separate Loopback Addresses

which are different to the Local Loopback Address

that we're discussing now, routers and switches do support the

Loopback Address of 127.x.x.x which allows us to verify

the TCP/IP Stack but Loopback interfaces

on routers and switches are very useful for other things

and are used by routing protocols such as OSPF