Class B subnetting

Please could someone help explain this 2 me, tried n tried but stil dont get it.

I understand the basic concept.For example, given an IP address of 165.5.0.0 and needin to create 6 subnets, i would borrow 3 bits and the subnet numbers would go up in increments of 32 i.e 165.5.32.0, 165.5.64.0 etc. Thats fine, i have no problem working out the range of hosts addresses of each subnet from here.

Here is my problem tho; say i have an IP address of 172.20.2.130 and subnet mask 255.255.255.128. This tells me that 9 bits have been borrowed but wot i dont understand is how the subnet numbers increment. value of last bit borrowed is 128 (from first bit in last octet) but that does not give the expected amount of subnets so am unable to do anythin more with this!

Can anyone help explain to me how i would work out the correct increment of subnets using the information i hav supplied?

Any help would be greatfully appreciated

Someone back me up on this.

172.20.[1-255].0
172.20.[0-254].128

if you break down the last two octets (for class it seems to make a little more sense. so if you have a mask of 255.255.255.128 you are borrowing 1 bit from the last octect; all 8 from the third.

172.20.xxxxxxxx.x0000000 x = subnet portion
172.20.00000000.10000000 (172.20.0.128) will be the first subnet
172.20.00000001.00000000 (172.20.1.0) will be the second
172.20.00000001.10000000 (172.20.1.128) will be the third and so on.

hope that helps a little

Thanks very much for the replies, this has helped me fully understand subnetting!

Many thanks