About a fire wall
The private network is located next to a public network. Only the connection with the Internet passes a fire wall and is controlling all traffic here.
A firewall is the special computer called gateway, and it is designed so that through and a cracker cannot access only specific traffic in a private network. Like a password, while a system administrator manages correctly and guarantees a private network, you have to set up a fire wall so that the network user can access.
In most networks, "the fire wall (fire wall)" is established between the private network and the public network. A fire wall buffers at once all the information that is the gateways between networks and can be exchanged in networks, and it is missing from the sieve.
A fire wall is that is, like the block for protecting, although a cracker and other users trespass upon a private network. However, it can let the information from a network administrator pass.