Tip & How-To about Computers & Internet
NetBIOS is a Windows File and Print Sharing protocol, and it uses TCP/UDP ports 135, 137,138, and 139.
* TCP/135 - RPC: This port is potentially quite dangerous. Remote Procedure Calls are requests from one machine to another for service. The RPC service acts as something of a facilitator, or go-between, between the client making the request and the machine being asked for service, i.e. a request is made to this "end-point mapper service" and then a port is allocated dynamically to the service being requested. This is similar to the RPC functionality found in the Unix world, and although it's not technically a "file sharing" port, it ties heavily into Windows networking in general.
* UDP/137 - NetBIOS Name Service: This port is used to attain name resolution for NetBIOS. Think of it as NetBIOS's version of DNS or ARP. It's simply a way to use something you have, make a query, and get something you want in return. For NetBIOS it's from a NetBIOS name to an IP, for DNS it's a DNS name to IP, and for ARP it's from IP to hardware address.
* UDP/138 - NetBIOS Datagram Service: This port primarily allows the SMB browser service to populate the browse lists seen when using "Network Neighborhood".
* TCP/139 - NetBIOS Session Service: This is perhaps the most known Windows port of all, as it is used to transfer files over TCP. This is both the port that NULL Sessions are established over and the port that file and printer sharing takes place on. If you are considering restricting access to ports on your Windows machine, this one needs to be on the top of the list.
NOTE: By disabling NetBIOS, all existing file shares must be shared using the IP address, rather than the NetBIOS Name, or else the share won't work.
How to disable NetBIOS in Windows 2000:
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