NetBIOS

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Contents 1 NetBIOS 2 NetBIOS Naming 3 NetBIOS Sessions 4 Quick Review 5 Answers:

[edit section] NetBIOS

NetBIOS, or Network Basic Input/Output System, allows for session-layer communication on the OSI model. NetBIOS is primarily concerned with two functions: naming and starting/stopping NetBIOS “sessions.” Since NetBIOS is not actually a networking protocol (it's an API) it is not routable and therefore nodes are only visible to other nodes within the same subnet. It also provides for an unreliable NetBIOS datagram service which is rarely utilized these days (and probably not on your Network+ exam). So, we will cover the two basic aspects of NetBIOS: naming and sessions.

[edit section] NetBIOS Naming

NetBIOS names are 16 bytes in length but usually consist of 15 characters, with the last being reserved for special purposes. You are probably familiar with the “15 character limit” if you have any experiences with naming PC’s – almost all operating systems require the PC name to be 15 characters in length or less. All NetBIOS names resolve to one or more IP addresses. If a NetBIOS name resolves to a single IP address (that is, if the relationship is said to be “one-to-one”), it is called a Unique Name. If the name resolves to more than one computer, it is said to be a Group Name. The Network+ exam tests you specifically on NetBIOS naming in Microsoft Windows networks. Name resolution is an important feature of a NetBIOS network; after all, how would you know which NetBIOS names correspond to given IP addresses? NetBIOS name resolution is handled through several means.

A broadcast is simply a request to all nodes on a network to resolve a given name. Think of it as calling out someone in a crowd. Yelling “Is Anthony Parks here?” may result in someone who identifies himself as Mr. Parks to turn around and proclaim, “Yes, I am Anthony!” Alternatively, perhaps nobody in the crowd bears that name, and the request may be forwarded to other people. Similarly, in a NetBIOS network, broadcasts are sent to all nodes, asking for a response if a computer recognizes the name as its own.

However, broadcasts can be cumbersome and boggle down a busy network – imagine the amount of noise created if everyone is asking for someone in a crowded room! To resolve this problem, several centralized NetBIOS name resolution services exist, including:

• DNS: The most common medium of name resolution. DNS stands for “Domain Name Service” and operates on the concept of “domains,” or specific, named hosts or networks that can account for sub-domains. DNS actually is deserving of its own section, so we will move on for now.
• NetBIOS Name Server: Stores NetBIOS names in a central location for easy name resolution. Returns an IP address based on a NetBIOS name.
• WINS Server: Microsoft’s version of NetBIOS name server. What you have to know about for the exam is that it utilizes a primary and optional secondary WINS server for redundancy and is considered wider in features than an ordinary NetBIOS name server.

Microsoft Windows in particular allows you to save NetBIOS name entries in a file.

• LMHOSTS file: Has NetBIOS name entries for the local area network.
• HOSTS file: Has NetBIOS name entries for remote network hosts as well as local ones.

[edit section] NetBIOS Sessions

The Network+ exam is far more concerned with NetBIOS naming, but here are some points to remember about NetBIOS sessions:

• Use TCP Port 139
• Provides Error Detection and Recovery
• Session Layer of OSI model
• Many Microsoft services utilize NetBIOS sessions

[edit section] Quick Review

1. A user wishes to connect to ACCOUNTING on a Microsoft network without a WINS server. ACCOUNTING is on the same subnet as the user’s computer. Which file should the user configure?

a. HOSTS

b. LMHOSTS

c. CONF

d. WINCONF

e. None of the Above

2. A user complains that he cannot communicate via NetBIOS to a remote host behind an NAT firewall. Which of the following courses of action would allow for connection?

a. Use NetBEUI instead of NetBIOS

b. Enable DHCP at the Server level

c. Utilize NetBIOS naming

d. Forward (open) TCP Port 139

e. Enable Loopback

[edit section] Answers:

1. LMHOSTS allows for the manual configuration of NetBIOS name resolution for local area network hosts. The answer is B.

2. The only option that would allow for communication through the NAT device is D, opening up TCP 139 and allowing the connection. The answer is D.