Even without subnetting hosts on the Internet or any other IP network are assigned a network number.
Network numbering allows a group of hosts (peers) to communicate efficiently with each other. Hosts on the same network may be computers located in the same facility or all computers used by a workgroup.
Multi-homed hosts containg multiple network adapters, can belong to multiple networks, but each adapter is assigned exactly one network number.
The table below illustrates the default numbering scheme for Class A, B, and C networks.
C networks.
In general, a network address uses the leftmost byte of its hosts' addressing if the hosts fall within the Class A range, the leftmost two bytes for hosts in Class B, and the leftmost three bytes for hosts in Class C. This algorithm is applied in practice through the use of a network mask.
The above table shows the decimal representation of the default network masks that is commonly used by network operating systems. Note that the decimal value '255' corresponds to one byte that has all bits set to one (11111111).
| Class | Host address range | Network address | Default mask |
| A | 0.0.0.0 - 127.255.255.255 | x.0.0.0 | 255.0.0.0 |
| B | 128.0.0.0 - 191.255.255.255 | x.x.0.0 | 255.255.0.0 |
| C | 192.0.0.0 - 223.255.255.255 | x.x.x.0 | 255.255.255.0 |
In general, a network address uses the leftmost byte of its hosts' addressing if the hosts fall within the Class A range, the leftmost two bytes for hosts in Class B, and the leftmost three bytes for hosts in Class C. This algorithm is applied in practice through the use of a network mask.
The above table shows the decimal representation of the default network masks that is commonly used by network operating systems. Note that the decimal value '255' corresponds to one byte that has all bits set to one (11111111).
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