Supernetting Question

Johnny Johnson

I have a question about supernetting, and was wondering if anyone could help me out.

A router connecting two small test labs has failed and needs to be replaced. The first lab has 10 computers and is assigned the network ID 192.168.21.0 with a subnet mask of 255.255.255.0. The second lab has 25 computers with the network ID 192.168.23.0 and a subnet mask of 255.255.255.0. Instead of replacing the failed router, you recommend that the two subnets be combined into a single supernet. What subnet mask would you use? What IP addresses are valid for the new supernet?


Thanks in advance,
Johnny Johnson

sprkymrk

Is there a reason they need to use seperate IP subnets? If there are no security and/or management requirements to seperate them, just combine them all on the same IP range and don't worry about supernetting them. You're only using 35 IP Addresses, so there is no real need to supernet 2 class C's when you can just use a single class C.

rwwest7

255.255.224.0 should do the trick.

rwwest7

Valid host range would be 192.168.1.2 - 192.168.31.254

rwwest7

rwwest7 wrote: »

Valid host range would be 192.168.1.2 - 192.168.31.254

If anyones bored, could they double check this for me. It just doesn't seem correct, I think I brain farted today.

tim100

Johnny Johnson wrote: »

I have a question about supernetting, and was wondering if anyone could help me out.

A router connecting two small test labs has failed and needs to be replaced. The first lab has 10 computers and is assigned the network ID 192.168.21.0 with a subnet mask of 255.255.255.0. The second lab has 25 computers with the network ID 192.168.23.0 and a subnet mask of 255.255.255.0. Instead of replacing the failed router, you recommend that the two subnets be combined into a single supernet. What subnet mask would you use? What IP addresses are valid for the new supernet?


Thanks in advance,
Johnny Johnson

You can use:

192.168.20.0
255.255.252.0

IP address range:

192.168.20.1 - 192.168.23.254

Johnny Johnson

tim100 wrote: »

You can use:

192.168.20.0
255.255.252.0

IP address range:

192.168.20.1 - 192.168.23.254

Tim,

Would you please explain how you came up with the answer? Thanks!

tim100

Johnny Johnson wrote: »

Tim,

Would you please explain how you came up with the answer? Thanks!

You have two network addresses:
192.168.21.0
255.255.255.0

192.168.23.0
255.255.255.0

In order to supernet these networks you have to take away subnet mask bits from the third octet. If you take away 2 bits this leaves subnet mask 255.255.252.0. Since you removed two bits this tells you that you can have 4 supernets. My method to find this out is by looking at the rightmost bit which is 4 but you can also look at the remaining zeros in the third octet and do (2^N) the N being your zeros. Your network addresses are 192.168.21.0 and 192.168.23.0 so looking at the fact that you can have 4 supernets you can start to figure out which supernet range your network would belong to. Again the rightmost bit helps in figuring this out. The rightmost bit is 4. You add this 4 as you figure out the supernet ranges:
192.168.0.0
192.168.4.0
192.168.8.0
192.168.16.0
192.168.20.0 - Your networks fall in this range
192.168.24.0
and so on....

This is my method of doing it but there are many other methods which may be easier for each individual.

sprkymrk

Again I need to ask - why supernet? You normally do that when you have 2 conditions:

1. You cannot subnet for some reason.
2. You need to add more hosts than a single subnet allows.

Johnny Johnson

sprkymrk wrote: »

Again I need to ask - why supernet? You normally do that when you have 2 conditions:

1. You cannot subnet for some reason.
2. You need to add more hosts than a single subnet allows.

Good point. In a normal situation, you probably wouldn't. But the question states that you are not going to replace the router, and is asking for a supernet to be used.

Johnny Johnson

tim100 wrote: »

This is my method of doing it but there are many other methods which may be easier for each individual.

Tim, thanks for the explanation. I think I'm starting to get the hang of this!

sprkymrk

Johnny Johnson wrote: »

Good point. In a normal situation, you probably wouldn't. But the question states that you are not going to replace the router, and is asking for a supernet to be used.

Right, that's one of the two conditions. Unless my math is really bad, 10 computers + 25 computers = 35 computers. You can have 254 on a single class C subnet. No supernet needed. Put them all on one subnet or the other. Much more standard practice and no fancy schmancy subnet mask needed. Supernets are really an exception to the "normal" way of subnetting. In other words, if you don't have a compelling reason to do it, then don't.

So was this a test question of some sort, or a real life situation? If the former, then forget what I said. I was under the impression this was a situation you were experiencing.

Johnny Johnson

sprkymrk wrote: »

So was this a test question of some sort, or a real life situation? If the former, then forget what I said. I was under the impression this was a situation you were experiencing.

It was just a question in a textbook I'm reviewing. I appreciate your input though.