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quick subnetting question

T

TJN23

Golden Member
hey all, i'm just learning about subnetting and i have a question

·Given: 223.13.40.30 and you need 2 hosts per subnet. Find: A. What is the subnet mask? How many subnets are there? What is the network address for the given IP Address? What is the broadcast address for the given IP Address?

My answer so far:
The subnet mask is 255.255.255.192/26
There are 4 subnets

I can't seem to figure out the network and broadcast addresses however...

any help is appreciated,

thanks in advance
 
If you search google, it's simple to find an IP calculator tool, usable on the website or you can download one. 🙂 It may be "cheating", but it can also help you to learn so that eventually you can do it in your head. Most of the network people I've known keep a cheat sheet nearby anyway, for those times when the brain just doesn't work.

Your answer for the subnet isn't right, since the /26 isn't part of the subnet. 🙂 It is CIDR notation and is only added to the network address, and is used to "shorthand" the subnet so you don't write the subnet as well.

I've never taken a "class" so that I'd run into a question like this on a quiz, so maybe I'm just missing it, but I don't see how you can determine any of the answers just on the basis of one IP address. Are we to presume that the "given" is the entire Class C network containing that IP?

In that case, your subnet mask would be 255.255.255.248 for each subnet (or a /30) because you have a block of 4 IPs: network, broadcast, 2 hosts, for every subnet. There are 64 subnets (256 addresses divided by 4 per subnet). The network address for the given IP is 223.13.40.28 and the broadcast address is 223.13.40.31. 29 and 30 are the host addresses.

A /26 would result in 4 subnets of 64 addresses each.
 
a /30 mask is 255.255.255.252 (four addresses, one all zeros, one all ones, two "usable" by convention).

I know that's what you meant, but wanted to correct it before anyone got confused.

FWIW

Scott
 
Originally posted by: Lord Evermore
If you search google, it's simple to find an IP calculator tool, usable on the website or you can download one. 🙂 It may be "cheating", but it can also help you to learn so that eventually you can do it in your head. Most of the network people I've known keep a cheat sheet nearby anyway, for those times when the brain just doesn't work.

Your answer for the subnet isn't right, since the /26 isn't part of the subnet. 🙂 It is CIDR notation and is only added to the network address, and is used to "shorthand" the subnet so you don't write the subnet as well.

I've never taken a "class" so that I'd run into a question like this on a quiz, so maybe I'm just missing it, but I don't see how you can determine any of the answers just on the basis of one IP address. Are we to presume that the "given" is the entire Class C network containing that IP?

In that case, your subnet mask would be 255.255.255.248 for each subnet (or a /30) because you have a block of 4 IPs: network, broadcast, 2 hosts, for every subnet. There are 64 subnets (256 addresses divided by 4 per subnet). The network address for the given IP is 223.13.40.28 and the broadcast address is 223.13.40.31. 29 and 30 are the host addresses.

A /26 would result in 4 subnets of 64 addresses each.
Click to expand...

He is given sufficient information to determine the answers. An IP address and the number of possible hosts per subnet should allow you to determine the answers to the questions.
 
How is he given all the information he needs? You have an IP address, and the number of hosts. Okay, so you know what size subnet that IP has to be in. But how does that define "how many subnets there are"? You can't determine that without knowing what size network allocation the IP is part of. The IP is obviously a Class C address, but they're not necessarily required to consider classful routing, so they could only be referring to a /28 for all we know. Perhaps it's ALWAYS assumed on that sort of test that when referring to any allocation that isn't specifically subnetted, the reference is to the entire Class allocation, but as I said, I don't know what the conventions are for this sort of test.

This one problem, on its own without any preconceptions about how questions are formulated, does not have enough information to answer all the parts of the question. The number of subnets can't be defined without a definition of the total allocation size.
 
actually, the allocation size is given by default that it is a class C address. A subnet is a "sub network" from the actual network class. I know we don't care about classfull addressing anymore, but by the book the answer to that question is 64 subnets.

So..

right off the bat 2 hosts per subnet = a /30. Meaning there are 256/4 = 64 subnets

223.13.40.30 is in the 223.13.40.28 network providing .29 and .30 as host addresses. .31 is the broadcast because the host portion of the address is all ones, but I think Lord Evermore already covered tha.
 
thanks so much guys....i just learned all this stuff and class today so now all these replies make sense 🙂

Tim (trying to grow out of the networking noob stage)
 
Originally posted by: Lord Evermore
How is he given all the information he needs? You have an IP address, and the number of hosts. Okay, so you know what size subnet that IP has to be in. But how does that define "how many subnets there are"? You can't determine that without knowing what size network allocation the IP is part of. The IP is obviously a Class C address, but they're not necessarily required to consider classful routing, so they could only be referring to a /28 for all we know. Perhaps it's ALWAYS assumed on that sort of test that when referring to any allocation that isn't specifically subnetted, the reference is to the entire Class allocation, but as I said, I don't know what the conventions are for this sort of test.

This one problem, on its own without any preconceptions about how questions are formulated, does not have enough information to answer all the parts of the question. The number of subnets can't be defined without a definition of the total allocation size.
Click to expand...

I do believe the given information was simplified by the poster. I just have a feeling that the original question probably tells you the size of the subnet (2 hosts per subnet).
 
Yes, it does say 2 hosts per subnet in the original post. But as far as I can tell, there is no definitive indication that an entire Class C is being referred to. Given the lack of specification on that point, one must assume the entire block, but there isn't a sure proof of that. From that question alone, one can't be aware whether the class in question uses classful routing as the basis of training or not, or whether this question was part of a particular segment of the class that ignores class.
 
Originally posted by: Lord Evermore
Yes, it does say 2 hosts per subnet in the original post. But as far as I can tell, there is no definitive indication that an entire Class C is being referred to. Given the lack of specification on that point, one must assume the entire block, but there isn't a sure proof of that. From that question alone, one can't be aware whether the class in question uses classful routing as the basis of training or not, or whether this question was part of a particular segment of the class that ignores class.
Click to expand...

If the question doesn't specify whether it's classful IP or classless IP and that is all the information provided, I am sure whoever wrote the question leave it up to you to figure out that you can determine the answers using classful routing.
 
No, the truer answer would be "Given the data and assuming classful addressing is in use, there are blah blah..."

That way, you take care of BOTH situation.
 
You also have to think.. do you want the true answer or do you want the answer as given by the author or the certification test 🙂

Cisco and Microsoft disagree on all sort of network design issues..

You need to learn what they are asking and what they are expecting as an answer.
 
spidey: subnetting isn't limited to classful situations. You can take a /26 (which is a subnet obviously) and subnet it further. The question could have referred to any size address block.

It's not "truer" to include the assumption of classful routing in the answer, it's only just as true. But it would be an actual answer rather than an avoiding of the question. Wonder if you can get extra points for pointing it out. 🙂

Microsoft seems to disagree with pretty much everyone about network, from what I hear about their certification exams. 🙂 Heck, they seem to disagree with actual experience on their Windows questions.
 
Ok, guys...?

One thing you can count on.

On a certification test where they are asking a question about subnetting, classful or otherwise, if they DON'T specify a subnet mask or prefix & length (a.b.c.d/x), then you can safely assume that they mean for you to use the default SN mask for whatever class IP address you are using.
 
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