Calculating Subnet Mask

[Nimbus]

Question about subnetting, I know the process of borrowing host bits from an octet to create subnets and how to calculate the subnet mask for the desired number of subnets.

I have seen questions in the MSpress book that asks you to calculate the subnet from a given network or host address?? can someone advise me on how this is calculated

For example, say if I had a network destination of 172.16.8.0 - how do I calculate the mask?

Thanks in Advance

[JonGlory]

You would have to assume the question is asking for the default subnet mask for the ip address stated unless it says otherwise.

The default for a 172 is 255.255.0.0 which is a class B

The defaults are class A = 255.0.0.0 with IP numbers from 1.0.0.0 - 126.0.0.0
Class B 255.255.0.0 IP range 128.0.0.0 - 191.0.0.0
Class C 255.255.255.0 Ip range 192.0.0.0 - 223.0.0.0

No 127.0.0.0 network, this is reserved

Hope this helps

[Nimbus]

Originally Posted by JonGlory

You would have to assume the question is asking for the default subnet mask for the ip address stated unless it says otherwise.

The default for a 172 is 255.255.0.0 which is a class B

The defaults are class A = 255.0.0.0 with IP numbers from 1.0.0.0 - 126.0.0.0
Class B 255.255.0.0 IP range 128.0.0.0 - 191.0.0.0
Class C 255.255.255.0 Ip range 192.0.0.0 - 223.0.0.0

No 127.0.0.0 network, this is reserved

Hope this helps

Sorry for the bad explanation, I am aware of the above. What I am really asking is that if 172.16.8.0 was a network address from a network with a non-default mask. Is it posible to work out what the mask is?

[JonGlory]

If all you are given is a network address and a told its a non default subnet mask, then no, I doubt you can figure out the subnet mask. I may be wrong.

IMO you would need to know the number of hosts or networks that the given ip and sn mask can support to work it out.

i.e the following network address "172.16.8.0 " supports 16 hosts, what is the subnet mask.

Answer 255.255.255.240

[r.h.lee]

Originally Posted by Nimbus

Question about subnetting, I know the process of borrowing host bits from an octet to create subnets and how to calculate the subnet mask for the desired number of subnets.

I have seen questions in the MSpress book that asks you to calculate the subnet from a given network or host address?? can someone advise me on how this is calculated

For example, say if I had a network destination of 172.16.8.0 - how do I calculate the mask?

Thanks in Advance

Nimbus,

Which "MSpress book" are you referring to?

[Tinus1959]

Originally Posted by Nimbus

Sorry for the bad explanation, I am aware of the above. Waht I am really asking is that if 172.16.8.0 was a network address from a network with a non-default mask. Is it posible to work out what the mask is?

It could be several. Easiest way is to write down the (in this case) thirth octet in binairy. 00001000
The most right '1' here is the border for the subnetmask, so that would give you 11111000 which is 248.

To be correct, also 252 would be valid for network 8 (the first network would be 4, 8 would be the second), and 254 (here 8 would be the fourth network with 2, 4 and 6 as the first three), 255 (as the eighth network), 255.128, 255.192, 255.224 and so on and so on.

[JonGlory]

Kinda defeats the purpose of having a subnet mask, if you can determine the network and host part of an ip unless you are using defaults.

[Tinus1959]

Originally Posted by JonGlory

Kinda defeats the purpose of having a subnet mask, if you can determine the network and host part of an ip unless you are using defaults.

No it does not. If you read my reaction again, you will see that I start with "Could be several". Just having the network address could give you a clue, but the subnetmask gives you certainty.
What's more, it also tells the computer exactly what the network address is.

[Tinus1959]

Originally Posted by JonGlory

If all you are given is a network address and a told its a non default subnet mask, then no, I doubt you can figure out the subnet mask. I may be wrong.

IMO you would need to know the number of hosts or networks that the given ip and sn mask can support to work it out.

i.e the following network address "172.16.8.0 " supports 16 hosts, what is the subnet mask.

Answer 255.255.255.240

There is no subnetmask which supports exactly 16 hosts. A network with mask 255.255.255.240 supports a maximum of 14 hosts.

[JonGlory]

Originally Posted by Tinus1959

No it does not. If you read my reaction again, you will see that I start with "Could be several". Just having the network address could give you a clue, but the subnetmask gives you certainty.
What's more, it also tells the computer exactly what the network address is.

You are guessing though and given that the network address 172.16.8.0 could be subnetted 255.255.248.0 - 255.255.255.254, that would be a very lucky guess if you got the question right.

Only if defaults sub net masks were used, would the router know the network address.

Originally Posted by Tinus1959

There is no subnetmask which supports exactly 16 hosts. A network with mask 255.255.255.240 supports a maximum of 14 hosts.

255.255.255.240 = 16 host, 14 are usable, but if subnet zero is used then 16 are usable.

You could reword the question

i.e the following network address "172.16.8.0 " has to support at least 16 usable hosts, what is the subnet mask.

Answer 255.255.255.224


Any question on an exam will give more information than the question posted by the author of the thread.

[Nimbus]

Originally Posted by Tinus1959

It could be several. Easiest way is to write down the (in this case) thirth octet in binairy. 00001000
The most right '1' here is the border for the subnetmask, so that would give you 11111000 which is 248.

To be correct, also 252 would be valid for network 8 (the first network would be 4, 8 would be the second), and 254 (here 8 would be the fourth network with 2, 4 and 6 as the first three), 255 (as the eighth network), 255.128, 255.192, 255.224 and so on and so on.

Thanks Tinus,

Thats is a great explanation, I never thought of looking at the third octet of the network address in that way before when trying to figure out the subnet address

I have dug up the question that confused me originally, I assume the subnet mask is 248 here also. Using the same reasoning - the subnets go in 8 with 248 being the most right bit in 11001000.

This question has an exact answer though as it has an end to the address pool.

Using Calculator, determine which subnet mask you must assign to the
address pool 207.46.200.0–207.46.207.255


255.255.248.0

Thanks to everyone who replied

[Tinus1959]

Originally Posted by JonGlory

You are guessing though and given that the network address 172.16.8.0 could be subnetted 255.255.248.0 - 255.255.255.254, that would be a very lucky guess if you got the question right.

Only if defaults sub net masks were used, would the router know the network address.

I suggest you reread the chapter on subnetting.

255.255.255.240 = 16 host, 14 are usable, but if subnet zero is used then 16 are usable.

No it has not 16 hosts, it has 16 addresses and only 14 of these can be used for hosts.

You could reword the question

i.e the following network address "172.16.8.0 " has to support at least 16 usable hosts, what is the subnet mask.

Answer 255.255.255.224


Any question on an exam will give more information than the question posted by the author of the thread.

You are correct here, but that was not the original question.

[JonGlory]

Originally Posted by Tinus1959

I suggest you reread the chapter on subnetting.

My subnetting is more than fine tbh, why go reread something i already know?

Originally Posted by Tinus1959

No it has not 16 hosts, it has 16 addresses and only 14 of these can be used for hosts.

I suggest you read this
http://www.cisco.com/en/US/tech/tk64...80093f18.shtml

Originally Posted by Tinus1959

You are correct here, but that was not the original question.

At least Im right here lol, what was the original question again, oh yeah, "how do you correctly guess a subnet mask"

[Tinus1959]

Originally Posted by JonGlory

My subnetting is more than fine tbh, why go reread something i already know?

Maybe you did not understand?

I suggest you read this
http://www.cisco.com/en/US/tech/tk64...80093f18.shtml

Read it. They are talking about subnets there, not about hosts. And when mentioning host ranges there they specificly leave out the 0 address and the 255 address.

At least Im right here lol, what was the original question again, oh yeah, "how do you correctly guess a subnet mask"

Funny. I suggest you reread the original question to.

[JonGlory]

You're right enough with the host Tinus, got mixed up with address there

But the original question is guessing a subnet mask

I.e network admin needs 256 networks with 256 addresses

Subnets ip 172.16.8.0 with 255.255.255.0