Subnetting

[mainodin87]

Hi Team,

What valid host range is the IP address 10.255.207.175/20 a part of?

Answer: 10.255.192.1 through to 10.255.207.254

i know the concept but is there any simple and quick solution to find the first valid host address, because the IP in question is 10.255.20.175, how to guess the IP will be starting from 10.255.192.0 for this range.

Hope you got what am trying to ask.

Thank You,

Mainodin

[mainodin87]

what about subnet's because when i calculate it with subnet calculator it is showing

for 10.100.25.2 with subnet mask 255.255.255.192

total host = 64 and total subnet = 262144

for 192.168.25.2 with subnet mask 255.255.255.192

total host = 64 and total subnet = 4

how is it ?

[Masoud Pourshabanian]

Caculator is calculating the subnets base on the classful perspective.

10.100.25.0/26 is one subnet in 10.0.0.0/8 which is a class A subnet

26-8= 18 then 2^18 probably gives you 262144 subnets

192.168.25.0/26 is one subnet in 192.168.25.0/24 which is a class C subnet

26-24  gives 2^2=4 subnets

Classfull concept is not used anymore

Hope it helps,

Masoud

[mainodin87]

Then what is the concept of class C subnetting with class A address?

[Masoud Pourshabanian]

If the first octet is between 0-126, it is a class A subnet and prefix is /8

2^24 host

examples

10.0.0.0/8

125.0.0.0/8

90.0.0.0/8

If the first octet is between 128-191, it is a class B subnet and prefix is /16

examples

129.16.0.0/16

129.18.0.0/16

150.209.0.0/16

2^16 hosts

If the first octet is between 192-223, it is a class c subnet and prefix is /24

examples

220.8.1.0/24

192.18.2.0/24

195.127.0.0/24

2^8 hosts

You can divide classful subnets to smaller subnets by borrowing bit from host portion and giving to prefix or network portion.

I agree with you. Classful and classless are confusing. You should use your intution to answer those quetions if classful or less is not mentioned.

Hope it helps,

Masoud

[Joseph W. Doherty]

If the first octet is between 0-126, it is a class B subnet and prefix is /8

Laugh, I'm not alone making typos - 0..126 would be a class A.

Since I've been using binary, 1st 3 bits indicate Class A, B or C.

0xx = Class A

10x = Class B

110 = Class C

[Masoud Pourshabanian]

:)

You are right. I copied from my second sentence and pasted it. Then forgot to change B to A.

I make these kinds of mistakes a lot.

Thanks for mentoning. I will edit it.

Masoud

[mainodin87]

LOLz .. Joseph

so what will be the good answer calculating it on basis of provided subnet mask i.e 255.255.255.192 or with class of IP address used ?

[mainodin87]

LOLz .. Joseph

so what will be the good answer calculating it on basis of provided subnet mask i.e 255.255.255.192 or with class of IP address used ?

[Joseph W. Doherty]

As Masoud has already noted, Classful addressing isn't much used anymore, so just calculate using one of the methods already posted.

[Masoud Pourshabanian]

In real world network, forget about classful. In exam, it depends on the questions, but usually classless. I will be back in 4 hours and discuss it with you.

Masoud

[Masoud Pourshabanian]

Let me give you some examples.

1- 192.168.1.0/25 is given to a network administrator. How many sunbets can network administrator create with using /28 prefix and how many hosts can be assigned to each subnet.

28-25=3   2^3=8.  8 subnets will be created.

32-28=4   2^4=16  16 hosts in each subnet.

As you see the main network(prefix or subnet) is given and the question is about subnetting. in this case, you do not need to consider classful subnets.

2-  I have seen many crazy questions like this which does not make any sense.

How many subnets and hosts per subnet can you get from the network 172.30.0.0/24

If you check the question, there is no information about changing the subnet or main prefix or main network. In this type of questions, you need to consider classful subnets.

172.30.0.0/16 is a class B subnet . We are changing 16 to 24. Now it looks like the first question.

24-16=8   we can create 2^8 = 256 subnets,

32-24=8   we can have 2^8 = 256-2 hosts per subnet

If you give me some example, I can tell you which approach you need to take.

Masoud

[mainodin87]

Hey Masoud,

That was bit confusing but a very good example, will get back to you if anything is required, thanks for your precious time :)

Regards,

Mainodin

[Masoud Pourshabanian]

My pleasure. I agree with you about confusion with classful and classless. Let me know if encounter any types of questions which you need to discuss. Do not worry. After some practice, you will calculate it in your head very fast.

Masoud

[Masoud Pourshabanian]

edited