# IP subnetting

#### Have a question or want to start a discussion? Post it! No Registration Necessary.  Now with pictures!

•  Subject
• Author
• Posted on
obtained ?

How many subnets are gained by subnetting 172.17.32.0/20 into multiple/
28 subnets?

They answer is 256, but I can not figure out how to come up with that

Thanks for the help

Re: IP subnetting
RIckH wrote:

If you keep 172.17.32.0/20 as is, you have just one big network (of course).

If you use a /28 mask, then you have the following subnets (relative to the

172.17.32.0/28      .00100000.0000 0000  (last two octets, space at /28)
172.17.32.16/28     .00100000.0001 0000
172.17.32.32/28     .00100000.0010 0000
172.17.32.48/28     .00100000.0011 0000
...
172.17.32.240/28    .00100000.1111 0000  (16 subnets so far)

then:

172.17.33.0/28      .00100001.0000 0000
...
172.17.33.240/28    .00100001.1111 0000  (other 16 subnets )

and so on until

172.17.47.0/28      .00101111.0000 0000
...
172.17.47.240/28    .00101111.1111 0000

So, you have 16*16 subnets (eg, 256) if you use a /28 mask.

You can also see this very quickly if you think that you gain 8 bits for the
subnet part (/20 -> /28), thus you have 2^8 = 256 subnets.

Ok, strictly speaking, you "gain" 255 subnets (you had one, and now you have
256), but I hope you got the idea.

Re: IP subnetting

Thanks, That makes perfect sense now.

Re: IP subnetting
RIckH wrote:

If you keep 172.17.32.0/20 as is, you have just one big network (of course).

If you use a /28 mask, then you have the following subnets (relative to the

172.17.32.0/28      .00100000.0000 0000  (last two octets, space at /28)
172.17.32.16/28     .00100000.0001 0000
172.17.32.32/28     .00100000.0010 0000
172.17.32.48/28     .00100000.0011 0000
...
172.17.32.240/28    .00100000.1111 0000  (16 subnets so far)

then:

172.17.33.0/28      .00100001.0000 0000
...
172.17.33.240/28    .00100001.1111 0000  (other 16 subnets )

and so on until

172.17.47.0/28      .00101111.0000 0000
...
172.17.47.240/28    .00101111.1111 0000

So, you have 16*16 subnets (eg, 256) if you use a /28 mask.

You can also see this very quickly if you think that you gain 8 bits for the
subnet part (/20 -> /28), thus you have 2^8 = 256 subnets.

Re: IP subnetting

Hi,

2^(28-20) = 256 subnets (with ip subnet-zero)

172.17.32.0/20        172.17.32.0-172.17.47.255
172.17.32.0       (16 subnets)
172.17.32.0-172.17.32.15
172.17.32.16-172.17.32.31
....
....
172.17.32.240-172.17.32.255

172.17.33.0     (16 subnets)
172.17.33.0-172.17.33.15
172.17.33.16-172.17.33.31
....
....
172.17.33.240-172.17.33.255
.....
.....
172.17.47.0    (16 subnets)
172.17.47.0-172.17.47.15
172.17.47.16-172.17.47.31
....
....
172.17.47.240-172.17.47.255

Regards,
Gabriele

Re: IP subnetting
I found a site that does a really good job at explaining this  (In My humble
oppinion)

Site:  http://www.firewall.cx /

the actuall explanation: http://www.firewall.cx/ip-subnetting-intro.php

Cheers,
the_Druid