Another subnet question

Admiral Akmir

How many subnets are possible with a Class B address and a subnet mask of 255.255.255.128?

This popped up on a practice quiz I was taking on a website, I picked 512 and they said it was wrong, and that the correct answer is 512. Why?

9 bits borrowed for subnetting, 2^9 = 512. If they were asking about hosts, then I could understand 510 as an answer, because of the broadcast address and network identifier, but this just doesn't make sense to me.

ande0255

Older sites will probably contain these type of legacy answers, as that is the old way of thinking I do believe (minusing 2 from both hosts and subnets), so the issue might be that the material was never updated.

Geek1969

It is asking for number of subnets.... not number of hosts.

Bits


/16

17

18

19

20

21

22

23

24

25



Networks

1

2

4

8

16

32

64

128

256

512



Hosts


65536

32768

16384

8192

4096

2048

1024

512

256

128



Beginning with a standard class B (/16) and adding 9 bits to end with a /25

xnx

As above, number of subnets is not calculated in the same way as number of usable hosts

bugzy3188

Look up zero subnets...as ande mentioned the first and last subnets of a network were at one time not allowed to be used in most (maybe all?) routing protocols and thus if the website is an older one may reflect this.

bugzy3188

As a quick aside...the CCNA does require knowledge of zero subnets