The concept of network actually not accurate, precise to define, in fact, it is the subnet.
Say belong to different network segments, in fact, it is a different subnet mask.
IP: network number (net-id) + host ID (host-id), a logical network number is used to identify the network, network host number used to identify a host.
A host with at least one IP address, and the IP address is unique in the whole network, if a host with two or more IP addresses, the host belongs to two or more logical networks, typically used as a router.
Same host can directly access each network number, a different number of host network need to access each other through the router.
Enough to use the IP address, subnet designers to add the field.
A means (or unit) to apply to the IP address of this unit is the network number net-id, and the number of hosts behind the host-id assigned by the unit, the unit for all of the hosts use the same network ID. When many hosts and the distribution of a unit in a large geographical area, often need some bridge (rather than the router, the router connection because the host has a different network number), these hosts are interconnected. But the disadvantage of many bridges, such as easily lead to broadcast storms and when a network failure is not very easy to isolate and management.
In order to facilitate the management of the host unit, this unit may be divided into a number of relevant sub-host, the first several bits as "the subnet number field", still behind the rest of the host and the host ID number field with the IP address. Such a router can be used according to the present interconnection between the sub-units, thereby facilitating management.
Subnetting is a matter for the unit inside, the outside can not see this division in the unit. From the outside, this unit still has only one network number. When an external packet enters the range of the unit, the unit is then routed router based on subnet, and finally the destination host. If the unit according to geographical location to host subnetting, then the management will be much more convenient.
With three subnet mask corresponding to the address, each router receives a packet, it first checks the IP address of the network number in the packet. If this is the network, from the routing table to find the next address to forward it out. If this network, and then check the IP address subnet number. If the subnet is present, this is the same packet forwarding; subnet if present, can be isolated in accordance with the host ID from what port the packet to the host.