3 Replies Latest reply: Jul 5, 2009 6:33 PM by Paul Stewart - CCIE Security

IP Subnet Zero Command?

Hi everyone,

Can someone explain IP Subnet Zero with an example. If 172.25.0.0 has been divided into eight equal parts and ip subnet zero is on the router, why are the increments of 32? why is the subnet mask 255.255.224.0

I understand it has subnet mask 255.255.248.0 with increments of 8, where I'm I wrong?

• 1. Re: IP Subnet Zero Command?

Can someone explain IP Subnet Zero with an example. If 172.25.0.0 has been divided into eight equal parts and ip subnet zero is on the router, why are the increments of 32? why is the subnet mask 255.255.224.0

I understand it has subnet mask 255.255.248.0 with increments of 8, where I'm I wrong?

I'm not entirely sure I understand your question, but I'll give it a go.

If you take the class B 172.25.0.0 and want to break it into 32 equal parts, you would use the 255.255.224.0 mask.

There are 256 possible subnets in each octet (don't forgot, in networking, 0 counts as a number): 256/8=32.  Which renders 32*256=8192 possible hosts per subnet

Remember, you use the high and low address of each subnet for the network and broadcast address, so you'd actually have 8190 hosts per 255.255.224.0 subnet.

You can use the magic number method, and subtract the 'interesting octect', that which isn't 255 or 0, from 256, and quickly determine the increments.  This method is easier for most than learning the binary method.

The breakdown would be like this: 255.255.224.0

172.25.0.0 - 172.25.31.255 (This is your subnet zero)

172.25.32.0 - 172.25.63.255

172.25.64.0 - 172.25.95.255

172.25.96.0 - 172.25.127.255

172.25.128.0 - 172.25.159.255

172.25.160.0 - 172.25.191.255

172.25.192.0 - 172.25.224.255

172.25.192.0 - 172.25.255.255 (This would be the broadcast)

If you were to use a 255.255.248.0 mask, you would be breaking the same Class B, but into 32 segments of 8 subnets, rather than 8 segmants of 32 subnets.  8*256=2048 addresses per subnet. (2046 hosts)

172.25.0.0 - 172.25.7.255 (subnet zero)

172.25.8.0 - 172.25.15.255

....

172.25.240.0 - 172.25.247.255

ip subnet zero will let use utilized the entirety of your IP range.  If you're in a situation where you need to use no ip subnet zero (RIP v1, IGRP, legacy equipment), then you would not be able to use the Zero subnet, or the broadcast subnet.

So in the case 255.255.224.0, instead of having 8 subnets of 8192 addresses (65536 addresses total), you'd have 6*8192=49152.  Meaning you lose more than 16,000 addresses.

For 255.255.248.0, you'd have 30*2048 (61440), losing 4096 addresses.

Hope this helps.

• 2. Re: IP Subnet Zero Command?

Hi

This is my guess

172.25 is a class B address therefore there are 16 bits in the network and 16 bits in the hosts

They want to subnet it into 8 subnets  so LOG2(8) = 3 or 2*2*2 = 8 so we have 3 bits for the subnet and 16-3 = 13 bits for the host

Now a octet has 8 bits   and 2*2*2*2*2*2*2*2 = 256   so 256 / 8 = 32

Therefore subnet mask is 256-32 = 224 so 255.255.224.0

The subnets are 172.25.0 to 172.25.31.255

172.25.32.0 to 172.25.63.255

...

172.25.(256-32).0 to 172.25.(256-32+32-1).255

Regards Conwyn

• 3. Re: IP Subnet Zero Command?

ip subnet-zero doesn't change the size of the subnets, but it does allow you to use the first and last subnet.  So for your example, you have the following subnets.

172.25.0.0 subnetted to 255.255.224.0 (or /19)

172.25.0.0/19 (First) <--

172.25.32.0/19 (2nd)

172.25.64.0/19 (3rd)

172.25.96.0/19 (4th)

172.25.128.0/19 (5th)

172.25.160.0/19 (6th)

172.25.192.0/19 (7th)

172.25.224.0/19 (Last)  <--

Without subnet zero, the first and last subnetworks are permitted for use.