Figuring out Subnet ID and Host ID from an IP address

Question

I have an exam practice question which reads: a class B network node has an IP address 10.10.155.59 and subnet mask 255.255.248.0. What's the subnet ID and host ID in decimal?
I know the answer is 38 and 827 so it's not homework help - would just love a simple explanation of how it got there. Thanks for any help :)

Answer 1

I think this is what you mean? Yet I think your 38 is incorrect.

10.10.155.59 = 00001010.00001010.10011011.00111011

So a class B network is /16 and you have a /21 network. So you would get this:

00001010.00001010 - 10.10 = Network ID (16 bits)
10011 - 19 = Subnet ID (5 bits)
01100111011 - 827 = Host ID (11 bits)

Also have a look here.

Answer 2

You have to check that in which octet the change is going to occur, so in your question it is 3rd octet so take the 3rd octet (155) & change it to binary then and (multiply) it with the subnet mask /21 in which two octets are already completed (/16) just the 3rd octet is there in which the change is occurred means some bits of 3rd octet are on & some are off.

3rd octet binary is 11111000 & and (multiply) it with the 155 binary 10011011 whatever the answer will be that will be considered the network ID of the given IP