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This is one of multiple videos discussing IP subnetting.

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I've received a number of questions asking me to explain various subnetting scenarios and this is one

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of them.

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So can the PC on the left configure it with this IP address 10 .1.248.1 slash 20 ping the PC

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on the right with IP address 10.1.192.2 slash 20.

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In this example, I'm using Cisco routers as my PCs and I'm doing that because it's important that you

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learn the Cisco CLI. 

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However, the subnetting behavior will be the same on Cisco versus a PC but it gives us a lot of options

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to troubleshoot and test our subnetting scenarios.

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So this is the IP address configured on gigabit 00, show run interface gigabit 00 shows us the IP address

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configuration and show IP interface gigabit 00 shows us the IP address inside the notation.

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So that's the address inside a notation and that's the address configured in dotted decimal notation.

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I'll do the same here show run interface gigabit 00 there's the IP address of PC 2 or router 2 acting

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as PC 2 dotted decimal notation and show IP interface gigabit 00.

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And what I'll do is only include the IP address of 10.

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So there's the IP address inside the notation.

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I could do it this way which shows all the output but it's nice to have that output as well as this

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output on the same screen.

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I'll do the same here show run interface gigabit 00 show IP interface gigabit 00 and I'll include 10.

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So there's the IP address inside the notation and a dotted decimal notation.

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So the question again can these two PCs ping each other.

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So let's test it.

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Ping 10.1.248.1

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notice the pings are failing ping 10.1.192.2, pings are failing.

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Why are they failing?

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The answer is the PCs are in different subnets.

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They connected physically to each other and we can see that by using the command show CDP neighbors

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notice router1 is connected to router 2 on gigabit 00 and router 2 is connected back on gigabit

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00.

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So they're definitely connected to each other physically but they can't ping each other.

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To help explain, I've taken the IP addresses in decimal notation and converted them to binary.

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So this is the binary representation of 10.1.248.1 

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Here's the binary representation of 10.1.192.2 slash 20 means 20 binary ones in the subnet

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mask.

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Here's the subnet mask with 20 binary ones.

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I typically don't have a space between your binary bits but I've just added that to make it easier to

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read.

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So here's the first octet, second octet, third octet

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But notice four binary ones in the third octet and four binary zeros.

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The same is shown here

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slash 20.

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So are these two devices in the same subnet.

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The answer is, No

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Because notice the network portion is different on these two devices.

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I've highlighted the network portion in red, the host portion as shown in blue. For devices to be in

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the same subnet the network portion or subnet portion needs to be the same on both devices, notice here

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the subnet portion is different.

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So they are in different subnets and won't be able to ping each other unless you put a router between

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them or change the subnet mask.

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What would you change the subnet mask to, to allow them to ping each other?

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And the answer is make the subnet portion this which means that the host portion is now changed

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to that.

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So this is the host portion

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this is the network portion, network portion is now the same.

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That equates to 8 plus 8, 16 17 18 binary bits which equates to 255.255.192.

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So previously that was the IP address on this side and that was the IP address on the right-hand side.

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So we would change the IP address to 10.1.192.2, 255.255.192.0 and on this side interface

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gigabit00, IP address 10.1.248.1, 255.255.192.0.

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So same IP address all I've changed is the subnet mask.

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So can the router on the right acting as our PC ping the router on the left acting as a PC.

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And the answer is, Yes it can.

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It dropped the first ping while doing an ARP but notice device on the right can ping the device on the

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left and the device on the left can ping the device on the right.

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And that's just because we changed the subnet mask to slash 18 and put the devices in the same subnet.

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I hope you enjoyed the video if it's been of benefit to you please like it and please subscribe to my YouTube

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channel.

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I wish you all the very best.