Subnetting valid hosts.

karilz

How do you calculate these, pls by explaining one or two? Since i do not understand.

valid host addresses for a subnet? (Choose three.)
• 10.1.12.79/28
• 10.1.12.113/28
• 10.1.12.32/28
• 10.1.11.5/27
• 10.1.11.97/27
• 10.1.11.128/27

j-man

What exactly don't you understand about this? What have you done to try to figure this out?

The key to these types of questions is to figure out the block size. A /28 indicates that the first four bits of the last octet are all set to 1 so the subnet mask would look like:

11111111 11111111 11111111 11110000

Counting from left to right you have 128 64 32 and 16. The last bit set to 1 tells you the block size so in this case it's 16.

Now count up in increments of 16

0
16
32
64
80
96
112
etc

You should know that a host cannot be the subnet number or the broadcast address.

So 10.1.12.113/28 is the only valid host out of the first three listed. 10.1.12.79/28 is a broadcast address and 10.1.12.32/28 is a subnet address.

Come on back and let us know which are the remaining valid host addresses.

If you have more questions, please ask.

karilz

I know what prefix is, but isn't valid host about counting from the host bits from right against left like: 2^5 = 32-2 = 30 valid hosts?
I can't get it work, because at the end if you do prefix & ANDing it all gets zero on host bits, so results get the same. This is what makes me confusing.
Or is it I'm misunderstanding something here, and yep i did try. I guess my methods wrong.


IP adr: 10 . 11 . 1 . 5/27
00001010.00000001.00001011.000|00101
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.000|00000 = 10.1.12.0

IP adr: 10 . 1 . 11 . 97/27
00001010.00000001.00001011.011|00001
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.011|00000 = 10.1.11.96

IP adr: 10 . 11 . 1 . 128/27
00001010.00000001.00001011.100|00000
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.100|00000 = 10.1.11.128

instant000

karilz wrote: »

I know what prefix is, but isn't valid host about counting from the host bits from right against left like: 2^5 = 32-2 = 30 valid hosts?
I can't get it work, because at the end if you do prefix & ANDing it all gets zero on host bits, so results get the same. This is what makes me confusing.
Or is it I'm misunderstanding something here, and yep i did try. I guess my methods wrong.


IP adr: 10 . 11 . 1 . 5/27
00001010.00000001.00001011.000|00101
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.000|00000 = 10.1.12.0

IP adr: 10 . 1 . 11 . 97/27
00001010.00000001.00001011.011|00001
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.011|00000 = 10.1.11.96

IP adr: 10 . 11 . 1 . 128/27
00001010.00000001.00001011.100|00000
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.100|00000 = 10.1.11.128

Your first one should be 10.1.11.5/27 and 10.1.11.0
Your last one is 10.1.11.128/27

I'm assuming that's just a typo, no big deal now, but something to watch out for, when you have to deal with these later.

You're only showing how to figure out what the subnet is, given an address and a mask, above.

If you want to know how many hosts can be in a subnet, then yes, it is 2^x -2, where x = number of host bits.

If you want to know what host addresses can be valid in a subnet, you figure out a few things.

1. the subnet number
2. the broadcast number
3. valid hosts are all the numbers inbetween those two.

What text are you using to study? It should cover this, to some degree.

j-man

karilz wrote: »

IP adr: 10 . 11 . 1 . 5/27
00001010.00000001.00001011.000|00101
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.000|00000 = 10.1.12.0

You are on the right track. What you figured out is what subnet that host belongs to. The next step is to find the broadcast for that subnet. To do that using the method above do the following

00001010.00000001.00001011.000|00101
11111111.11111111.11111111.111|00000

---

00001010.00000001.00001011.000|00000 <- Take the host bits to the right of the pipe and set them to all 1 so you get the following

00001010.00000001.00001011.000|11111 <- Now convert that last octet back to decimal and you have your broadcast which is 31

Using that information you know that the first valid host is 10.1.11.1, the last is 10.1.11.30, the broadcast 10.1.11.31.

instant000 is right. It sounds like you are getting the two questions How many subnets and hosts will /X give you and find the valid host addresses for a given subnet combined/confused.

Also check out the thread Subnetting Made Easy.