Whats the difference in subnetting IP address and Network address?

Please Explain why?
Using Network Address 172.16.30.10 with mask /18 subnet to obtain 500 host for each subnet.
Answer was 172.16.30.10 /23

Using IP Address 172.16.30.10 with mask /18 . subnet to obtain 500 host for each subnet
Answer was 172.16.30.0 /23

Why is that for network address the .10 in the last octet remains and for IP address there is no .10 ?

Cisco has a good explaination at the link below:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

Hi,
An subnetting IP address is a logically visible subdivision of an IP All computers that belong to a subnet are addressed with a common, identical, most-significant bit-group in their IP address.A network address is a unique identifier for a computer on a network. When set up correctly, computers can determine the addresses of other computers on the network and use these addresses to send messages to each other.And If you want find the IP address of the computer you can follow the steps : Go to -Start -CMD -IP config /all .this will show you the IP address and the subnet IP address of the computer.And to find the network IP address visit the site http://www.ip-details.com .