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Another special type of address is the Local Loopback Address

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

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to itself and this helps verify or make sure that

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the TCP/IP stack is installed correctly on that machine

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A typical Loopback IP Address is 127.0.0.1 but remember please

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that anything in the 127 range is deemed a Loopback Address

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

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of IPv4, a 127 address is a Class A Address

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Class A Addresses have 16 million odd host addresses

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and by chossing this address range for the Loopback Address

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we lost the use of potentially 16 million IP Addresses

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Host Addresses cannot be allocated as Public IP Addresses

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on the Internet thus making the IPv4 Address space smaller than

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

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

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the same mistake again

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

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and this will give me the IP Address of my machine

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

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on my PC, so I could ping 10.0.0.6

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and as you can see the ping succeeds

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I can also however ping my Loopback Address

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So I could ping 127.0.0.1

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and notice the ping also succeeds

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but don't forget anything in this ranges supported

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so 127.127.127.127 is also a Loopback Address

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What about 127.1.2.3?

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

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is also a Loopback Address and anyone of those addresses

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could be used to test the TCP/IP Stack

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Now this machine has both IPv4 and IPv6 installed

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So here's my PC's IPv6 Address

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

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

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Now this video doesn't discuss IPv6 in any detail

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

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

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IPv6 Addresses are not wasted or to be more politically correct

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IPv6 Addresses are optimized more than they are in IPv4

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Please also note that routers and switches

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

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as the Loopback Address that we're discussing here

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

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Engineers will often refer to a routers Loopback Address

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

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

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127.0.0.1 but routers and switches can also be configured

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

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

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Interface Loopback 0, configured with an IP Address

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

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10.1.1.1 with a /32 mask configured on a Loopback Interface

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but Network Engineers may refer to the router's Loopback

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IP Address and they are referring to this IP Address

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

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

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is that routers and switches have separate Loopback Addresses

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which are different to the Local Loopback Address

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

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

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the TCP/IP Stack but Loopback interfaces

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on routers and switches are very useful for other things

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and are used by routing protocols such as OSPF