Subnetting....My Nemesis

control

Hi All,

I've been looking through our subnets at work and look to be using 172.16.0.0 /24

I can also see the following 172.17.246.32 /27

What are the available subnets / hosts ranges for that address above? (the/27) one.

I have no control over the network btw - just having a browse in AD, looking at the network address space and the /24 made sense, but then I got lost with the /27.

Anyone advie on the ranges for the above?

Thanks

Roguetadhg

172.17.246.32
255.255.255.224
usable hosts: 30
range: 172.17.246.32 - .63

carwashguy

I'm just getting into subnetting, I think it would be this.

172.17.246.0, .32, .64, .96, .128, .160, .192, .224

control

Thanks,

I also see there is 172.17.254.32 /27

I take it this is another valid range using the same principles as before?

SharkDiver

Carwashguy is right on the money.

You can use anything from 172.16.0.0 to 172.31.255.255 for Class B Private Addresses.

When you start with 172.17, you can have anything from 0 to 255 in the third octet.

Then, the /27 subnets in to 8 groups of 32, of which 30 IPs can be assigned to hosts in each group.

Each group would start with the IPs that CarWashGuy said:
.0, .32, .64, .96, .128, .160, .192, .224

But, of course, the first addresses that you can use are .1, .33, .65, .97, .129, .161, .193, and .225

control

So am I right in thinking that the 172.17.246.32 /27 address space would not crossover any I.P addresses from the 172.16.0.0 /24 subnets?

What would be the reasons for using the /27 subnet instead of just continuing with the /24? Is this to save/not waste addresses for smaller sites?

Just trying to get my head around WHY things are done.

Many Thanks

lrb

The /27 subnet is more than likely to have been selected to conserve IP addresses. A /27 provides 32 addresses (30 usable) and is a good choice for a subnet which has 10-20 hosts in it and is not likely to scale upwards quickly. This is the same reason that most engineers choose to use /30's or /31's on point to point links (we use /31's on GRE tunnels as per policy but I'm not the biggest fan of them) because there is only ever going to be 2 devices on the link so it makes sense to not waste addresses unnecessarily.

And your correct in thinking that these prefixes don't have overlapping addresses. The 172.16.0.0/24 prefix covers IP addresses from 172.16.0.0 to 172.16.0.255, and the 172.17.246.32/27 prefix covers IP addresses from 172.17.246.32 to 172.17.246.63.