How is this IP calculated?

[Zap]

I was asked to find the subnet, first and last assignable address, broadcast address, and address range of IP 197.110.145.18 /15. I converted the IP and subnet mask into binary.

11000101 01101100 10010001 00010010 : IP address
11111111 11111110 00000000 00000000 : Subnet mask

197.110.0.0 is the subnet that 197.110.154.18 resides on
197.110.0.1 is the first host address

Looking at the last octet in the IP and Subnet Mask, only the last bit is a 0 on both. If I convert that 0 to a 1 that would make the last address in the range 19, and the last assignable address would be 18. To check my work I used a subnet calculator and it really surprised me when it showed:

197.111.255.254 – last assignable address
197.111.255.255 – broadcast address

How did we get from 197.110 to 197.111? Can anyone show me that math so I can better understand this process?

[Biggles77]

Recheck the 2nd and 3rd octecs binary conversions.

[Zap]

Sorry, I did enter the binary string incorrectly. The result is still the same.

11000101 01101110 10100101 00010010 = 197.110.145.18
11111111 11111110 00000000 00000000 = 255.254.0.0 for the mask

I see one host bit (very last decimal value in the very last octet)

197.110.145.0 is the subnet the IP resides on.

*******************************************

11111111 11111110 00000000 00000000 = 255.254.0.0 Subnet mask
11111111 11111111 11111111 00000000 = 255.255.255.0 Default mask for a class C address.

If I calculated correctly, changing the host bits to 1's gives me a mask of 255.254.0.255. Please show me the math to find the last assignable address, range of IP address and how the network address becomes:

197.111.255.254 – last assignable address
197.111.255.255 – broadcast address

[Lisa]

Zap,

I wanted to let you know about our free TCP/IP training (http://www.trainsignaltraining.com/free-video-training/free-tcpip-networking-fundamentals-training-videos/).

Lesson 6 is about binary numbers and submnetting.

See if this helps.
Lisa

[greenmatter]

Thanks for the link!http://www.imgsnatch.com/img/w/N.gif

[Zap]

Yes, thank you for the link!