Subnetting Question

bbarrick

Could someone break this one down for me:

172.16.0.0 Network Address

255.255.255.128 Subnet Mask

Could have sworn the first method I studied said the Subnet ID's would be 172.16.0.0 and 172.16.0.128.
The broadcasts being 172.16.0.127 and 172.16.0.255.
The hosts being 172.16.0.1 - .126 and 172.16.0.129 - 172.16.0.254

This second method of subnetting is throwing me off a bit on this one subnet mask and giving me an entirely different answer.

smcclenaghan

I guess I"d have to see the question, but given 172.16.0.0 with mask 255.255.255.128, assuming no VLSM, then you'd have these networks:

172.16.0.0/25 = 172.16.0.0 through 172.16.0.127 (with 1-126 as usable hosts, 0 as network address, and 127 as broadcast)
172.16.0.128/25 = 172.16.0.128 through 172.16.0.255 (with 129-254 as usable hosts, 128 as network address and 255 as broadcast)

Not sure what you mean by second method of subnetting.

FloOz

172.16.0.0/25 would give you two networks. Your increment being 128 in the 4th octet

172.16.0.0 - 172.16.0.127
172.16.0.128 - 172.16.0.255

bbarrick

Here's the Lammle example:

172.16.0.0 Network
255.255.255.128

Subnets = 2^9 (counting the third octet and the one bit of the fourth octet) = 512
Hosts = 2^7 (the last 7 bits of the fourth octet) = 126

For valid subnets he's using the third octet as the magic number, 255(this is what throws me off).
So 265-255 = 1

Giving you 172.16.0.0, 172.16.0.128, 172.16.1.0, 172.16.1.128 etc...

So your first subnets hosts would be 172.16.0.1 to 0.126 with .127 being the broadcast.

Kind of threw me for a loop...

bbarrick

I can see Lammle's point, but I recall Odom saying that if the octet of the mask is 255 you copy the related octet of the ip address.

bbarrick

I've figured it out though, his explanation of that subnetting problem is what threw me off. It doesn't change at all, but he's technically lumping all those subnets together using 255 as the interesting octet. When he should have just said if 255 is in the mask octet you copy that number for the subnet id. It still gives you 512 subnets but it's because you can copy whatever number from 1 to 255 and it will give you two subnets each no matter what. He's acting as if the third octet is the interesting octet when it's really not.

smcclenaghan

You know, I thought about this after my original post.

I was wrong. I just assumed class C, but obviously 172.16 is class B, so you have to subnet all the way from 172.16.0.0/25 to 172.16.255.128/25.

You won't see a question like that. But you will see (all day long), how many /25 networks are in 172.16.0.0/16.