Subnet masks

[Bill]

Stumbled accross this on another forum and thought I'd make a stab at answering it. Keep in mind that I'm no teacher, so what makes sense to me, may not help another. I've added a link below to additional help.

Ok, we have an IP address of 150.170.110.120 which we know is a "B" class (255.255.0.0)

But the subnet mask given was 255.255.252.0

How did they get the .252??

Another example.....200.100.150.110 (Which is a "C" class, 255.255.255.0)

Yet the subnet given is 255.255.255.224.

How did they arrive at the .224?


Is this a matter of subnetting, or is this a CIDR issue.

Keep in mind that the subnet mask defines the network by defining, or segregating the network bits in an address from the host bits.

Class B subnet mask begins at 255.255 0.0. As you move down the heirarchy of subnets, the mask will further define lower level networks. The highest level Class B networks will use 255.255.0.0 as a subnet mask.

255.255.0.0 - Top level Class B Network = 11111111.11111111.00000000.00000000
255.255.128.0 - Second level subnet = 11111111.11111111.10000000.00000000
255.255.192.0.- Third level subnet = 11111111.11111111.11000000.00000000
255.255.224.0 - Fourth level  subnet = 11111111.11111111.11100000.00000000
255.255.240.0 - Fifth level  subnet = 11111111.11111111.11110000.00000000
255.255.248.0 - Sixth level subnet = 11111111.11111111.11111000.00000000
255.255.252.0 - Seventh level subnet = 11111111.11111111.11111100.00000000
255.255.254.0 - Eighth level subnet = 11111111.11111111.11111110.00000000

These are all Class B subnet masks.

255.255.255.0 - Top level Class C network = 11111111.11111111.11111111.00000000
255.255.255.128 - Second level subnet = 11111111.11111111.11111111.10000000

And so on for Class C.

Learn to Subnet

[Transk53]

Nice post there. Subnetting can look quite daunting sometimes, I used a Warehouse metaphor when I came accross it, rather than look entirely at the numbers.

[Bill]

I think the original poster had it set in their mind that Class B subnets could only be 255.255.0.0 and Class C could only be 255.255.255.0. I'm not sure if I really clarified anything or not.

[Transk53]

Yeah. Must admit that when I first read up on subnets, I did think that the address ranges were confined to that particular IP range. Not that I have dealt with Class B ranges though, but your seems clear to me.