Hi Carl,

It's not as difficult as u think it is, but every one might has his own calculations for subnetting, In the end, there is one standard for it, here is an example and explanation:-

lets say we have: 10.10.10.0/27 (that means it's a subnet of 32 IP's).

the available IP's range is from (10.10.10.1----10.10.10.30)

the ip 10.10.10.31 is considered broadcast ip.

the ip 10.10.10.32 is considered the next subnetwork.

so, now, we u spilit it up to /30s, it should be as follows:-

10.10.10.0/32 (that means it's a subnet of 4 ip's)

the available IP's range is from(10.10.10.1----10.10.10.2)

the ip 10.10.10.3 is considered broadcast ip

the ip 10.10.10.4 is the next subnetwork.

Now:

10.10.10.4/30 (that means it's a subnet of 4)

available ip's is (from 10.10.10.5-----10.10.10.6)

the ip 10.10.10.7 is considered broadcast ip (not usable)

the ip 10.10.10.8 is the next subnetwork.

And So On till we reach:

10.10.10.28/30 subnet

Available ip addresses (from 10.10.10.29-to-10.10.10.30)

the ip address 10.10.10.31 is considered a broadcast address (not usable)

the ip 10.10.10.31 is a next network ID (next subnetwork).

so simply, now we devided 10.10.10.0/27 to multiple 10.10.10.0/30s subnets as follows:

10.10.10.0/30

10.10.10.4/30

10.10.10.8/30

10.10.10.12/30

10.10.10.16/30

10.10.10.20/30

10.10.10.24/30

10.10.10.28/30

Note: the more lower /(bits) of subnet the more bigger lenght of usable IP's and vice verca

for example: Any ip address consists of 32 bits;

/32 highest bits means (255.255.255.255)(1 ip address available)

/30 high bits means (255.255.255.252)(2 ip addresses available)

/28 low bits meas (255.255.255.240) (14 ip addresses available)

/26 lower bits means (255.255.255.192) (62 ip addresses available).

And so on.....

pls rate the post if it helps,

Mohamed