Network Basic Input/Output System provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network.

<aside> ❗ As strictly an API, NetBIOS is NOT a networking protocol.

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In modern networks, NetBIOS normally runs over TCP/IP via the NetBIOS over TCP/IP (NBT) protocol. This results in each computer in the network having both an IP address and a NetBIOS name corresponding to a (possibly different) host name.

NetBIOS name

NetBIOS name of a domain joined computer

is 16 ASCII characters, however Microsoft limits the host name to 15 characters and reserves the 16th character as a NetBIOS Suffix. This suffix describes the service or name record type such as host record, master browser record, or domain controller record or other services.

The host name (or short host name) is specified when Windows networking is installed/configured, the suffixes registered are determined by the individual services supplied by the host. In order to connect to a computer running TCP/IP via its NetBIOS name, the name must be resolved to a network address. Today this is usually an IP address (the NetBIOS name to IP address resolution is often done by either broadcasts or a WINS Server – NetBIOS Name Server).

<aside> ℹ️ A computer's NetBIOS name is often the same as that computer's host name, although truncated to 15 characters, but it may also be completely different.

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NetBIOS services

NetBIOS provides three distinct services:

• Name service (NetBIOS-NS) for name registration and resolution.

In NBT, the name service operates on UDP port 137 (TCP port 137 can also be used, but rarely is).

The name service primitives offered by NetBIOS are:

• Add name : registers a NetBIOS name.
• Add group name : registers a NetBIOS "group" name.
• Delete name : un-registers a NetBIOS name or group name.
• Find name : looks up a NetBIOS name on the network.

IPv6 is NOT supported!

• Datagram distribution service (NetBIOS-DGM) for connectionless communication.

In NBT, the datagram service runs on UDP port 138.

The datagram service primitives offered by NetBIOS are:

- **Send Datagram** : send a datagram to a remote NetBIOS name.
- ****Send Broadcast Datagram : send a datagram to all NetBIOS names on the network.
- Receive Datagram : wait for a packet to arrive from a Send Datagram operation.
- Receive Broadcast Datagram : wait for a packet to arrive from a Send Broadcast Datagram operation.

• Session service (NetBIOS-SSN) for connection-oriented communication.

In NBT, the session service runs on TCP port 139.

The session service primitives offered by NetBIOS are:

• Call : opens a session to a remote NetBIOS name.
• Listen : listen for attempts to open a session to a NetBIOS name.
• Hang Up : close a session.
• Send : sends a packet to the computer on the other end of a session.
• Send No Ack : like Send, but doesn't require an acknowledgment.
• Receive : wait for a packet to arrive from a Send on the other end of a session.