Determining Network ID

Hi all, I have this /27 bit IP address (ie 235.85.105.64/27) which I think is a subnet address [pls correct me if im wrong] and Im trying to determine the network ID. I understand that to do this you carry out a bitwise AND of the IP (235.85.105.64) and the subnet mask (255.255.255.224). The network ID i obtain is 235.85.105.1 which i reckon is incorrect. anyone able to help me out here please? thanks in advance.

edit: Im not sure whether to use 255.255.255.0 for the AND operation or 255.255.255.224.

secondly will I be right to say the above address is a subnet address with 30 valid IPs even when it falls in the range of a class D (multicast address) - im a bit confused pls help?

The address that you have identified is the network ID portion. So 235.85.105.64/27 is the networkid giving valid address from;

235.85.105.65 - 235.85.105.94 = 30 host address

255.255.255.224
11111111.11111111.11111111.111 00000

If we look at the last octet

64 = 010 00000

The Host portion is all 0's therefore we are looking at a network ID. The first valid host address = 010 00001 The last valid host address = 010 11110 (because 010 11111 is the broadcast address)

thanks for your fast reply. am i right to say we can have 8 subnets (i.e. 000, 001......111). or 6 subnets without (i.e. 000 and 111) im a bit confused about this. any responses appreciated.

Yes, thats correct. You have 3 bits for the NetworkID which is therefore 2^3 = 8. As you said, normally you cannot use the first and last so you will often see 2^3-2=6

If you take a look at this post here , StarFire confirms that in todays routers that support Subnet 0, this is allowed but for CCNA, etc... you still need to include the -2 rule.

Cheers