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NetBIOS/NetBEUI - an explanation

Microsoft's networking protocols for its LAN Manager and Windows NT products.In order to properly describe NetBEUI, the transport protocol sometimes used for Microsoft networking, it is necessary to describe Microsoft networking in some detail and the various protocols used and what network layers they support. NetBIOS, NetBEUI, and SMB are Microsoft Protocols used to support Microsoft Networking. The NetBIOS stack includes SMB, NetBIOS, and NetBEUI.

NetBIOS Extended User Interface (NetBEUI) is a separate protocol from NetBIOS. It supports small to medium networks providing transport and network layer support. It is fast and small and works well for the DOS operating system but NetBEUI is not a routable protocol.
There are three methods of mapping NetBIOS names to IP addresses on small networks that don't perform routing:
  • IP broadcasting - A data packet with the NetBIOS computer name is broadcast when an associated address is not in the local cache. The host who has that name returns its address.
  • The lmhosts file - This is a file that maps IP addresses and NetBIOS computer names.
  • NBNS - NetBIOS Name Server. A server that maps NetBIOS names to IP addresses. This service is provided by the nmbd daemon on Linux.

System wide methods of resolving NetBIOS names to IP addresses are:
  • b-node - Broadcast node
  • p-node - Point-to-point node queries an NBNS name server to resolve addresses.
  • m-node - First uses broadcasts, then falls back to querying an NBNS name server.
  • h-node - The system first attempts to query an NBNS name server, then falls back to broadcasts if the nameserver fails. As a last resort, it will look for the lmhosts file locally.
  • NetBIOS name services use port 137 and NetBIOS session services use port 139. NetBIOS datagram service uses port 138.

To resolve addresses from names, a computer on a Microsoft network will check its cache to see if the address of the computer it wants to connect to is listed there. If not it sends a NetBIOS broadcast requesting the computer with the name to respond with its hardware address. When the address is received, NetBIOS will start a session between the computers. On larger networks that use routers, this is a problem since routers do not forward broadcasts, nor is NetBEUI a routable protocol. Therefore Microsoft implemented another method of resolving names with the Windows Internet Name Service (WINS). The following steps are taken to resolve NetBIOS names to IP addresses for H-node resolution on larger networks using TCP/IP (NBT):
  • NetBIOS name cache
  • WINS Server
  • NetBIOS broadcast
  • lmhosts file
  • hosts file
  • DNS server


NetBIOS over TCP/IP (NBT)

Since NetBEUI is not a routable protocol, Microsoft implemented NBT for larger networks. NetBIOS messages are normally encapsulated in NetBEUI datagrams, but when using NBT, they are encapsulated in TCP/IP datagrams. The NBT protocol is defined by RFC 1001 and RFC 1002.


NWLink

NWLink is Microsoft's implementation of IPX/SPX. NWLink will act as a transport mechanism for NetBIOS similar to the use of TCP/IP described in the NBT section above. NWLink is normally used to support medium networks and may be used where NetWare servers are present.


WINS
Windows Internet Name Service (WINS) is the Microsoft implementation of NetBIOS name service. Samba on Linux can be used as a WINS server. Computers configured to use WINS, when booted, contact the WINS name server and give the server their NetBIOS name and IP address. The WINS server adds the information to its database and it may send the information to other WINS servers on your network. When a computer that is configured to use WINS needs to get an address of another computer, it will contact the WINS server for the information. Without the use of a WINS server, NetBIOS will only be able to see computers on the unrouted sections of the local network. Does this mean a WINS server must exist in each routed section of the network? The answer is no. This is because WINS uses TCP/IP which is routable. Only one WINS server needs to exist on the network.