NetBIOS

NetBIOS was originally developed by IBM and Sytek as an API for client software to access LAN resources. NetBIOS has become the basis for many other networking applications. NetBIOS names are used to identify resources (e.g., workstations, servers, printers) on a network. Applications use these names to start and end sessions. NetBIOS names can consist of up to 16 alphanumeric characters. Clients advertise their name to the network. This is called the NetBIOS registration process and it is completed as follows:

1. The client broadcasts itself and its NetBIOS information when it boots up.
2. If there is another machine on the network that already has the broadcasted name, that NetBIOS client issues its own broadcast to advertise that the name is in use. Subsequently, the client that is trying to register stops all attempts to register that specific name.
3. The client finishes the registration process if there is no other machine with the same name on the network.

Cisco ASA supports NetBIOS by performing NAT of the packets for NetBIOS Name Server (NBNS) UDP port 137 and NetBIOS Datagram Service (NBDS) UDP port 138.

To enable NetBIOS inspection on the Cisco ASA, use the ip inspect netbios command.