IP Addresses - Network and Host ID

An IP address is a 32-bit number that uniquely identifies a host (computer or other device, such as a printer or router) on a TCP/IP network.
In TCP/IP wide area network (WAN) , the routers that pass packets of data between networks do not know the exact location of a host for which a packet of information is destined. Routers only know what network the host is a member of and  using the information stored in their routing table they determine how to get the packet to the destination host's network.

An IP address has two parts. The first part of an IP address is used as a network address, the last part as a host address.
Consider an IP address 192.168.123.133, splitting in to two parts
192.168.123 - Network
           .133 - Host
     or
0.0.0.133

Subnet Mask:
The subnet mask is used by the TCP/IP protocol to determine whether a host is on the local subnet or on a remote network.In TCP/IP, the parts of the IP address that are used as the network and host addresses are not fixed, so the network and host addresses above cannot be determined unless you have more information about a  32-bit number called a subnet mask.
Consider subnet mask 255.255.255.0 for our example, then converting to binary values:
192.168.123.133    -   11000000.10101000.01111011.10000101


255.255.255.0     -      11111111.11111111.11111111.00000000
 
Lining up the IP address and the subnet mask together, the network and host portions of the address can be separated:
11000000.10101000.01111011.00000000 - Network address
 
00000000.00000000.00000000.10000101 - Host address
 
The first 24 bits (the number of ones in the subnet mask) are identified as the network address, with the last 8 bits (the number of remaining zeros in the subnet mask) identified as the host address.

What is an IP Address?
IP Addresses-Classes
IP Addresses-Subnetting and Gateway