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 10

and by chossing this address range for the Loopback Address 11

we lost the use of potentially 16 million IP Addresses 12

Host Addresses cannot be allocated as Public IP Addresses 13

on the Internet thus making the IPv4 Address space smaller than 14

it may have been, in IPv6 a different addresses used for the 15

loopback ::1 and they made sure that they didn't make 16

the same mistake again 17

So here's an example, I can type ipconfig on my PC 18

and this will give me the IP Address of my machine 19

In this example it's 10.0.0.6 that's the Local IPv4 Address 20

on my PC, so I could ping 10.0.0.6 21

and as you can see the ping succeeds 22

I can also however ping my Loopback Address 23

So I could ping 127.0.0.1 24

and notice the ping also succeeds 25

but don't forget anything in this ranges supported 26

so 127.127.127.127 is also a Loopback Address 27

What about 127.1.2.3? 28

it also works, so anything in the range 127.x.x.x 29

is also a Loopback Address and anyone of those addresses 30

could be used to test the TCP/IP Stack 31

Now this machine has both IPv4 and IPv6 installed 32

So here's my PC's IPv6 Address 33

So I could ping 2001:20::2 which is my Local PCs IP Address 34

but I could also ping ::1 which is my IPv6 Loopback Address 35

Now this video doesn't discuss IPv6 in any detail 36

But it's worth fighting out that we have both an IPv4 Loopback 37

such as 127.0.0.1 as well as an IPv6 Loopback such as ::1 38

IPv6 Addresses are not wasted or to be more politically correct 39

IPv6 Addresses are optimized more than they are in IPv4 40

Please also note that routers and switches 41

also have Loopback Addresses and that's not the same concept 42

as the Loopback Address that we're discussing here 43

So be aware that in the field or in the real world 44

Engineers will often refer to a routers Loopback Address 45

and that's not the same as what we're discussing here 46

You've got the Local Loopback which is the Class A Address 47

127.0.0.1 but routers and switches can also be configured 48

with valid IP Addresses on what's called a Loopback interface 49

So a router or switch may have a Loopback Interface such as 50

Interface Loopback 0, configured with an IP Address 51

of 10.1.1.1/32, so in other words we have a Class A Address 52

10.1.1.1 with a /32 mask configured on a Loopback Interface 53

but Network Engineers may refer to the router's Loopback 54

IP Address and they are referring to this IP Address 55

and not 127.0.0.1, I will explain as I've mentioned in a moment 56

what a Network mask does but what I'd like to point out here 57

is that routers and switches have separate Loopback Addresses 58

which are different to the Local Loopback Address 59

that we're discussing now, routers and switches do support the 60

Loopback Address of 127.x.x.x which allows us to verify 61

the TCP/IP Stack but Loopback interfaces 62

on routers and switches are very useful for other things 63

and are used by routing protocols such as OSPF