listen for session request traffic on specific
ports and therefore cannot make use of port address translation
technology. Recall from previous curriculum that UDP and TCP
have roughly 65,535 associated ports each. It is highly
unlikely that a single client host accessing network resources
will legitimately need to use all of these ports at the same
time. Typically, a single host being used for normal Internet
activities will maintain around a dozen sessions through a NAT
router at any given time. Consider these typical Internet
browsing activities: - How many web pages are open at
the same time?
- Is email or newsgroup software
updating messages in the background?
- How many file
download sessions are active?
- Are any IM applications
running in the background?
- What background processes,
like DNS, are supporting other applications?
Because a single inside host does not require all of the ports
available on an outside address, NAT overload allows multiple
inside hosts to make use of the unused ports on a common
outside address. It does this by including the port numbers for
a given session in the translation table. Recall that a single
entry in a normal static or dynamic NAT table represents a
single host inside the network. In a table for NAT with
overload, a single entry now represents a single
transport-layer session. Figure shows a translation table with
entries for NAT with overload.
Content 6.1
Characteristics of Transport Layer Technologies
6.1.8 NetBIOS and NetBEUI NetBIOS and NetBEUI are a
pair of protocols that work together to provide easily
configured, broadcast-based networking service for computers
running the Microsoft Windows family of operating systems.
NetBIOS supports the following network services:
- Network name registration and verification
- Session establishment, termination, and management
- Reliable session data transfer (connection-oriented)
- Unreliable datagram data transfer (connectionless)
- Monitoring and management of network interfaces and
lower-layer protocols
One of the main advantages of
using NetBIOS and NetBEUI is that they are simple to configure.
The main failing of NetBIOS is that it uses a broadcast-based
non-hierarchical namespace, forcing it to rely on other network
layer protocols if used over a routed network. NetBIOS
Names
The NetBIOS name space is 16 bytes long. The
first 15 characters are the computer name defined by the user
during configuration. The last character is a hexadecimal digit
representing the type of data in the NetBIOS name and network
service using that data on a given machine. NetBIOS computers
advertise this information to show what services they can offer
to the remainder of the network. NetBIOS names can also be used
to identify a machine as being part of a workgroup or domain.
The 16th character is also used to identify a NetBIOS name as
being either unique as in a computer or user name, or group
such as a Microsoft Windows Workgroup or Domain name. Figure
shows the common suffixes for NetBIOS names and the Microsoft
networking services that use them. NetBIOS in Operation
There are three main stages in the operation of NetBIOS:
- Name registration
- Name discovery
- Name release
When a Windows NT computer starts
up, the various services running on the machine register
themselves using the unique NetBIOS name of the machine and the
appropriate hexadecimal character at the 16th byte position.
This process is called NetBIOS name registration and uses
either a broadcast to the local network or a unicast to a local
NetBIOS name server if one has been configured for the
computer. When one NetBIOS host needs to communicate with
another, it needs the NetBIOS name of the remote machine. This
name can either be input by the user or can be obtained from a
network browse list. The browse list is a list of network names
and services available on the network that is maintained by a
master browser machine. The browse list is built from the
NetBIOS information broadcast during the NetBIOS name
registration process during the start up of a machine. This
list is maintained by the master browser and is sent to one or
more backup browsers in the network. Opening Network
Neighborhood in Windows displays the browse list for the
network. Periodically, the NetBIOS host computer contacts the
master browser for a list of backup browsers from which one is
selected. The host computer then contacts the backup browser,
retrieves the browse list, and displays it to the user. If the
client cannot locate a master browser on the network, it
initiates a browser election to ensure a master browser and at
least one backup browser is elected in the network. The
computer browser service only supplies NetBIOS name
information. The NetBIOS name discovery process is what occurs
when a NetBIOS name needs to be resolved to a lower-level
network address. On a local network, this process is completed
using either broadcasts to the local network or using a unicast
message to a NetBIOS name server if present. The NetBIOS name
resolution process is discussed later in this section When an
application or networking service on a host is stopped, the
NetBIOS name for that service on that host is available for use
by another host. This process is called “NetBIOS Name Release”
and is the process that is used to remove services from the
network when a network host is shut down. NetBIOS Name
Discovery and Resolution
The operation of NetBIOS-based
networks is dependent on resolving NetBIOS names to lower-layer
addresses. Because resolving names to addresses is so
important, Windows computers use the following process to
efficiently check up to six sources of information when
attempting to connect to another computer: - Check the
local NetBIOS name cache
- Contact a NetBIOS name
server
- Broadcast the name resolution request locally
- Check the locally stored LMHOSTS file (Windows only)
- Check the locally stored HOSTS file (Windows only)
- Contact a Domain Name System server (Windows only)
- Windows computers maintain a local cache
of recently resolved NetBIOS names. This speeds up resolution
of names for computers that are connected to regularly.
- A NetBIOS Name Server (NBNS), normally a Microsoft Windows
NT server running the Windows Internet Name Service (WINS)
service, maintains name and address information for all
computers on the network. Windows computers configured to use a
NBNS server do a number of things during startup and shutdown.
During startup, the client computer requests its
own name from the name server to check that no other computer
on the network has the same computer name. Recall that NetBIOS
names must be unique on the network. If the client computer is
already registered in the network, the name address currently
being used by the client computer is returned. If the client
computer is new to the network, the server returns a message
indicating that the requested information is not found, and the
client computer registers its own name, address, and services
provided in the WINS database. This information remains in the
database until the machine is shut down when the client
computer requests its registration be released from the
database. - If the NBNS does not contain name resolution
information for the requested machine name, the Windows client
sends a local broadcast in an attempt to contact the remote
host directly. If there is no response, the Windows computer
checks locally stored files.
- Administrators of
Windows computers can configure them with files containing
hard-coded name-to-address resolution information. The first of
these files is called LMHOSTS (LAN manager hosts) and is a
remnant of older implementations of Windows networking.
- If a suitable entry is not found in the LMHOSTS file,
another locally stored file called HOSTS is checked. The HOSTS
file is similar to the LMHOSTS file and is a remnant of older