What is NetBIOS?

NetBIOS(Network Basic Input Output System)

When viewing network shares on a local area network, NetBIOS is being used from the remote computer to the host sharing the resources.

NetBIOS can supply some of the following information when querying a computer:

Host name, NetBIOS name, Domain, and most importantly Network shares.

While using NetBIOS on a network, it can communicate by either using sessions, which is connection oriented, or datagrams which is connectionless.

Using sessions, larger message sizes can be transmitted with error detection, but communication is only one to one.

The port for NetBIOS session communication is port 139.

However, with datagrams, communication is one to many but messages are smaller and don't have error detection.

Keep in mind that NetBIOS is not a Layer 3 protocol but merely an API to be used on top of TCP.