retransmission of all session service packets, and the dividing of the data stream over which the packets are transmitted into IP datagrams small enough to fit in link-layer packets. Sessions are closed by closing the TCP connection. Security vulnerabilities NBT exposes information and interfaces that are often appropriate for a LAN under an organization's administrative control, but which are not appropriate for a less trusted network such as the Internet. For example, the NetBIOS Name Service (NBNS), running over UDP or TCP port 137, allows any computer to register its hostname with other computers. An attacker could contact any host and claim that they are a particular service the host regularly contacts, such as a file server. This could result in a middleperson attack against listening hosts, and ultimately in the compromise of credentials used by the listening hosts to access network services over NBT. Tools such as NBNSpoof can be used to perform this attack. Exposure of NBT to the Internet also discloses, as a practical matter, that the host answering on NBT ports is running Windows. This can be used to better target malicious activity that might be specific to one operating system. Decreasing relevance in post-NT Client-Server Networks In relation to post-MS Windows 2000 / NT, client-server based networks, NetBIOS is effectively becoming a legacy protocol. NetBIOS was also developed for non-routable LANs. In most post year 2000 networks operating Windows 2000 or later, NetBIOS effectively offers backwards compatibility for network devices that predate compatibility with DNS. A central role of NetBIOS in Client-Server networks (and also those networks that have networked peripheral hardware that also predates DNS compatibility) is to provide name resolution to computers and networked peripherals. Further, it allows for such networked hardware to be accessed and shared and also enables the mapping and browsing of network folders, shares and shared printers, faxes, etc. In its primary capacity, it acts as a session-layer protocol transported over TCP/IP to provide name resolution to a computer and shared folders. To that end, Windows 2000-based, Client-Server networks - and later - do not require this insecure means of name resolving and addressing or navigating of network shares. Troubleshooting NetBIOS nbtstat The nbtstat command is a diagnostic tool for NetBIOS over TCP/IP. Its primary design is to help troubleshoot NetBIOS name resolution problems. The command is included in several versions of Microsoft Windows. There are several commands involved