Routing Summary // Help!

[mduong91]

Hi everyone, I'm new to the forums here. I'm currently studying Computer Networking Technology and we're using the Testout Corporation's materials to study. (We're studying at Renton Tech)

The class and myself are having a difficult time answering this question about what subnets to add, but especially why are the answers are the way they are. We'd appreciate the help! Thank you!

(the answer explanation, doesn't really help us... we really need help from anyone who could explain more simply)

[Dieselboy]

The question is correct but I think they give confusion by describing the range of addresses contained within the mask. See my attachment here. It makes much more sense to draw it out on paper which if you're learning, you should be getting into doing this as a habit. Then later you can take shortcuts and use a subnet calculator

I'll try and explain a bit better. Please excuse my poor handwriting on the attachment. This was my second attempt  I've never been able to make my handwriting neat, it just goes too slowly then I lose my thought train.

The mask is /28. You need a second /28 and need to be able to route to both using a summary. A summary route is a network and mask that includes the other networks. So the only way possible is to use a larger subnet (ie more hosts portion), which means a smaller subnet mask. If you draw this out like I have you will see that the .80 becomes .64 when you decrease the size of the mask to /27.

On my attachment, note 1 is showing where the mask was decreased to give us room to play. You should then be able to see that the 28th bit can now be turned on and off to create separate subnets with a /28 mask. The subnet we have (10.0.155.80/28) has the 28th bit turned on. So the only option we have is to turn it off which creates 10.0.155.64/28.

Note 2: with the /27 mask, the 28th bit is in the host portion of the subnet. This is why the /27 mask is a summary for those two subnets.
note 2 also shows how to work out the route. Basically, adding the bits of the network portion (128+64+32) gives 224. Remember we're only working in the last octet (after the dot which I marked as the 24th bit. Really, the bit is the 24th bit but I've found it easier to mark the dot as 24).

So it doesnt give only 224, it gives 255.255.255.224 which is the /27 mask.

So you'd need to change the static route from 10.0.155.80/28 to 10.0.155.64/27 which would include both the old and new subnets as described in the working out.


Whenever you get a problem like this, you should work out what you have to work with like I have drawn out with the 1's and 0's. At the top of the page I have drawn the dot as 24th bit and then marked the bits beyond that as 25, 26, 27, 28 and so on up to 32. Then directly underneath the 25 I have marked the octet values, 128, 64, 32 and so on. I'm going in to a lot of detail explaining subnetting here, you probably know all this already?

A subnet mask just shows the network and host boundary on the IP address you have and whether the IP address is a network address or a host address. Take 10.0.155.80 for example. It looks like an ip address to anyone on the internet. But in this router it's actually a subnet address and we can show that by drawing the subnet mask on top of it. The reason it's a subnet address is this key point: All of the bits in the host portion are ZERO! If any of the bits in the host portion were 1, then this would be a host address. (if all bits were 1 then it's a broadcast).

The other description / explanation they gave are about using a /26 mask. This would waste /28 subnets (ie they are being routed to B but not in use on B).

The last one "using a network of 10.0.155.80 for the summarised route is impossible when using a /27 or /26 mask" . Hopefully you should be able to explain why this is impossible now? Hint: draw what this looks like. So draw the .80 on paper then draw the subnet mask line (/27 and or /26) Why is this as a subnet impossible?

PS if you dont let me know why it's impossible then I'll be upset. 

[mduong91]

Hey Dieselboy,

Thanks for the help! Really appreciate the explanation and the pic you shared.
So.... Here's my understanding before I relay it to the rest of the peanut gallery.


1.) When the question states that the NEW subnet should use a 28-bit mask,
- I subtracted 16 from 80 in 10.0.155.*80*     
(because /28 the subnets are increments of 16)
- I'm left with 10.0.155.64/28       
(now with this info, moving on to the next part to replace the static route)
- I think I got this part down and digested in my brain.

2.) For the static route, I don't know where to begin to find why the mask is /27
- but my first clue is 10.0.155.64 and 10.0.155.80 from part 1
(need to include the old and new network)

- I understand /27 goes by increments of .32  , therefore..... 32 -> 64 -> 96

- so I know 10.0.155.64 in both /27 and /28 begin their subnet at .64

***now this part below, I need this part validated by an expert***

- If 10.0.155.64 /27 has a range of .64 through .95, it also includes 10.0.155.*80* (thus the old network)
- Being that it is in range, this would be the right answer?


3.) 10.0.155.80 /26 or /27 wouldn't work because the increments of the subnets in dotted decimal doesn't allow for .80?!

/26 = 0, 64, 128 ..... NO 80!
/27 = 0, 32, 64, 96 ..... NO 80!

(man... I get anxiety when having to answer, hahaha)

[Dieselboy]

I've never thought of it that way "the /28 mask goes in increments of 16" but if that helps you then it's fine

Let's try and explain it another way which might be easier to understand. You know a /24 mask right? 1.0.0.0/24 for example means that the last octet is hosts. So the range is 1.0.0.0 to 1.0.0.255. This can be easier because the last dot is the /24. So lets say that the "route" or rather the summary route is this 1.0.0.0/24 well then we can divide the this subnet into smaller subnets. The next smallest subnet is a /25. So you could create subnets of 10.0.0.0/25 and then the next one would have the 25th bit switched on which would make 10.0.0.128/25. Both of these /25 subnets are covered by the /24 mentioned earlier. There are no more /25 subnets other than these two that are covered by the /24.

Does that explain a bit better?


About the 3rd point you mention, you are correct! the .80 does not make a subnet. But the  key thing you need to be thinking of is that a .80 means that with a /26 or /27 subnet mask, there are HOST bits turned ON with a .80. So in this case a .80 is a host address and not a network address. If you try and configure a route in a network device like this it will tell you that you're wrong and the command will fail.

Given the same /24 example above, you cannot summarise the /25 subnet with another /25. Each of those /25 subnets are uniquely separate. This is why the summary is /24. Because with a /24 it "covers" the two /25. Another way of saying it is that the summary address with the /24 mask means that the two /25 subnets are hidden within the summary address. They are "hidden" because the /25 subnets are actually host addresses with a /24 mask. The route is saying "here's how to get to these hosts" and the hosts are within the range of 10.0.0."0-255". The route is locally significant to the that router device. On the router A, those two subnets are locally significant there.

Key points to remember:
A host address has one or more host bits turned on (given the subnet mask you will need to work out which part of the IP address is the subnet part and which part is the host part. Do what I did and draw the line to mark it, so it's easy to visually see. Then you can simply work out the subnet ID by making all the host bits a zero.

A subnet address has all host bits a zero.

A broadcast address has all host bits turned on, i.e. they are 1's for all the host bits.