Subnet

rivanfrank

Hi everyone,

I need help for this question?

The network 172.25.0.0 has been divided into eight EQUAL subnets. Which of the following IP address can be assigned to host in the third subnet if the ip subnet-zero command is configured on the router.

A. 172.25.78.243
B. 172.25.98.16
C. 172.25.72.0
D. 172.25.94.255
E. 172.25.96.17
F. 172.25.100.16

I know the answers are A,C and D because the third subnet range is from 172.25.64.1 to 172.25.95.254

What bothers me is, you cannot divide this network into 8 EQUAL subnets even though it is configured using ip subnet-zero.

172.25.0.0
172.25.32.0
172.25.64.0
172.25.96.0
172.25.128.0
172.25.160.0
172.25.192.0
172.25.224.0 <- this subnet is invalid for /19 (255.255.224.0) because if you add 32 the result is 172.25.256.0 which is wrong.

Am i correct or wrong? Please help me on this.

Thanks.

mella060

The 172.25.224.0 subnet is perfectly valid with a usable host range of 172.25.224.1 - 172.25.255.254 and a broadcast address of 172.25.255.255.

The 256 just signifies the start of the next subnet which would be 172.26.0.0, then 172.26.32.0 and 172.26.64.0 and so on

boredgamelad

If you study it a bit more carefully, you'll notice that 224-255 is 32 numbers, just like 0-31, 32-63, 64-95, 97-127, 128-159, 160-191, and 192-223. Each of the ranges stops one IP short of the next subnet.

rivanfrank

I see, I get it. Thanks.

rivanfrank

Thank You!