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NetBIOS over TCP/IP stats resolves NetBIOS names to IP Nbtstat is a diagnostic tool for NetBIOS over TCP/IP. It is included in several versions of Microsoft Windows. Its primary design is to help troubleshoot NetBIOS name resolution problems. Nbtstat -c: displays the contents of the NetBIOS name cache, the table of NetBIOS names and their resolved IP addresses. Nbtstat -n: displays the names that have been registered locally on the system. Nbtstat -r: displays the count of all NetBIOS names resolved by broadcast and querying a WINS server. Nbtstat -R: purges and reloads the remote cache name table.
Nbtstat -RR: sends name release packets to WINs and then starts Refresh. Nbtstat -s: lists the current NetBIOS sessions and their status, including statistics. Nbtstat -S: lists sessions table with the destination IP addresses. Looking Glass servers are computers on the Internet running one of a variety of publicly available Looking Glass software implementations. A Looking Glass server (or LG server) is accessed remotely for the purpose of viewing routing information.
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Essentially, the server acts as a limited, read-only portal to routers of whatever organization is running the LG server.citation needed Typically, publicly accessible Looking Glass servers are run by Internet service providers (ISPs) or Internet exchange points (IXPs). Near end crosstalk (NEXT) happens when a signal from a transmitter at one end of a cable interferes with a receiver at the same end of the cable result of crossed cables, crushed cables, improper striping near RJ-45 NEXT is a measure of the ability of cabling to reject crosstalk. Interference between two pairs in a cable is measured at the same end of the cable as the interfering transmitter. Crosstalk is undesirable. In crosstalk, the signals traveling through adjacent pairs of wire in twisted-pair cabling interfere with each other.
The pair causing the interference is called the 'disturbing pair,' while the pair experiencing the interference is the 'disturbed pair.' Channel NEXT is the NEXT value measured between one wire pair and another in the same cable; NEXT is measured at both ends of the wire. The NEXT value for a given cable type is generally expressed in decibels per feet or decibels per 1000 feet. NEXT value varies with the frequency of transmission. The higher the NEXT value, the greater the cable's ability to reject crosstalk at its local connection. Generally specifications for cabling (such as CAT 5) include the minimum NEXT values.1. Diagnose, implement STP, turn off service, segment network One of the best ways to troubleshoot a broadcast storm is to find out where the broadcasts are coming from.
It's most likely that they are all originating from multiple devices, but the only way you'd know is if you captured the packets themselves. So pull out your favorite protocol analyzer and capture some data from your network to see how many broadcasts are going across the network, and where are they coming from? Once you've identified the broadcasts and what devices are sending this, you can then determine if that broadcast is really necessary. Maybe it's a service that you can disable on a particular device or maybe you can modify the application so it sends unicast instead of broadcasts. If you find that you do have many different devices, they're all sending broadcasts, and there's no alternative, they must send these broadcasts out to operate, then maybe it's time to split the network into smaller pieces. In computer networking, the maximum transmission unit (MTU) of a communications protocol of a layer is the size (in bytes or octets) of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.).
Standards (Ethernet, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time. A larger MTU brings greater efficiency because each network packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means an improvement in bulk protocol throughput. A larger MTU also means processing of fewer packets for the same amount of data.
In some systems, per-packet-processing can be a critical performance limitation. However, this gain is not without a downside. Large packets occupy a slow link for more time than a smaller packet, causing greater delays to subsequent packets, and increasing lag and minimum latency.
For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer (and hence over most of the Internet), ties up a 14.4k modem for about one second. Large packets are also problematic in the presence of communications errors. Corruption of a single bit in a packet requires that the entire packet be retransmitted. At a given bit error rate, larger packets are more likely to be corrupt. Their greater payload makes retransmissions of larger packets take longer. Despite the negative effects on retransmission duration, large packets can still have a net positive effect on end-to-end TCP performance.1.
A smartjack may provide signal conversion, converting codes and protocols (e.g. Framing types) to the type needed by the customer equipment. It may buffer and/or regenerate the signal, to compensate for signal degradation from line transmission, similar to what a repeater does.
Smartjacks also typically provide diagnostic capabilities. A very common capability provided by a smartjack is loopback, such that the signal from the telephone company is transmitted back to the telephone company. This allows the telephone company to test the line from the central office, without the need to have test equipment at the customer site. Turns around telco signals to see if telco end is working correctly. A CSU/DSU (Channel Service Unit/Data Service Unit) is a digital-interface device used to connect a data terminal equipment (DTE), such as a router, to a digital circuit, such as a Digital Signal 1 (T1) line. The CSU/DSU implements two different functions. The channel service unit (CSU) is responsible for the connection to the telecommunication network, while the data service unit (DSU) is responsible for managing the interface with the DTE.
DSL and cable modems are CSU/DSUs because they convert from one type of digital signal to another. A CSU/DSU is the equivalent of the modem for an entire LAN.1.
FS Official 2017-11-03 You may have noticed that every piece of hardware on your local network has a MAC address in addition to the IP address. Except for switches which have switch MAC address, all devices that connected to the Internet have this unique identifying number, from desktop computers, laptops, cell phones, tablets to wireless security cameras, and even your connected refrigerator have a MAC address.
So, why does your network devices need two addresses to connect to a network? Isn’t an IP address sufficient? What exactly is that MAC address for? To put the MAC (Media Access Control) address in layman’s terms, you can think of the MAC address as your unique digital fingerprint, which is one of a kind in the world. A MAC Address is given by the manufacturer and it is embedded in the chip that allows your device to connect to a network.
For a network switch, it is likely to have many MAC addresses, since one MAC address is assigned to every interface on the switch. An Overall View of Mac Address A MAC address, also known as “hardware address” or “physical address”, is a binary number used to uniquely identify computer network adapters. Packets that are sent on the Ethernet are always coming from a MAC address and sent to a MAC address. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address. If the addresses match, the packet is processed, otherwise it is discarded. Traditional MAC addresses are 12-digit hexadecimal numbers.
The leftmost six hexadecimal digits of the address correspond to a manufacturer's unique identifier, while the rightmost six digits correspond to the serial number of the network interface card (NIC). MAC vs IP Address Relationship Initially it might seem that IP addresses and MAC addresses are redundant because both are unique identifiers of networked devices, but they actually serve different purposes, and are visible in very different ways. MAC operates at Layer 2 of the OSI model while IP operates at Layer 3.
MAC addresses are typically used only to direct packets from one device to the next device as data travels on a network. That means that the MAC address of your computer’s network adapter travels the network only until the next device along the way. If you have a router, then your machine’s MAC address will go no further than that. While when your computer wants to send a packet to some IP address x.x.x.x, then the first check is if the destination address is in the same IP network as the computer itself. If x.x.x.x is in the same network, then the destination IP can be reached directly, otherwise the packet needs to be sent to the configured router.
So do you see what’s going on? The MAC address just gets the data packet to the next device but the IP address is responsible for getting it to the ultimate destination. What Do Switches Use Mac Address For? Switches are unlike hubs or repeaters. A hub simply rebroadcasts every signal on every port to every other port, which (while inefficient and slow) is easy to create. A switch, on the other hand, intelligently directs traffic between systems by routing packets only to their proper destination.
To do this, it keeps track of the MAC addresses of the NICs plugged into each port. MAC addresses need to be unique or at least highly unlikely to be repeated for switches to identify different ports and devices, which is why manually setting a MAC address can have unexpected consequences in a switched network.
Switches usually have a bunch of MAC addresses reserved in its MAC address table. When forwarding a frame, the switch first looks up the MAC address table by the destination MAC address of the frame for the outgoing port.
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If the outgoing port is found, the frame is forwarded rather than broadcast, so broadcasts are reduced. How Do Switches Learn Mac Address?
Since a switch has some intelligence, it can build the MAC address table automatically. The following part will illustrate how a switch learns MAC addresses. There’s a switch in the middle and we have 3 computers around. All computers have a MAC address but they are simplified as AAA, BBB, and CCC. The switch has a MAC address table and it will learn where all the MAC addresses are in the network.
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Now, assuming Computer A is going to send something to Computer B: Computer A is going to send some data meant for computer B, thus it will create an Ethernet frame which has a source MAC address (AAA) and a destination MAC address (BBB). The switch has a MAC address table and here’s what will happen: The switch will build a MAC address table and only learns from source MAC addresses. At this moment it just learned that the MAC address of computer A is on interface 1. It will now add this information in its MAC address table. But the switch currently has no information where computer B is located. There’s only one option left to flood this frame out of all its interfaces except the one where it came from.
Computer B and computer C will receive this Ethernet frame. Since computer B sees its MAC address as the destination of this Ethernet frame it knows it’s meant for him, computer C will discard it.
Computer B is going to respond to computer A, build an Ethernet frame and send it towards the switch. At this moment the switch will learn the MAC address of computer B.
That’s the end of our story, the switch now knows both MAC addresses and the next time it can “switch” instead of flooding Ethernet frames. Computer C will never see any frames between Computer A and B except for the first one which was flooded. You can use the show mac address-table dynamic command to see all the MAC addresses that the switch has learned. One other point worth emphasizing here is that the MAC address table on the switch uses an aging mechanism for dynamic entries. If the MAC address of Computer A and B are not updated within their aging time, they will be deleted to make room for new entries, which means the frames between computer A and B will be flooded to Computer C again if A wants to transfer information to B. How to Configure Your Switch Mac Address Table? A MAC address table is maintained for frame forwarding, which can be dynamically learned or manually configured.
The former has been introduced in the previous text, and the next part will focus on how to configure the MAC addresses manually to adapt to network changes and enhance network security. Configuring Static, Dynamic, and Blackhole MAC Address Table Entries To improve port security, you can manually add MAC address entries to the MAC address table to bind ports with MAC addresses, fending off MAC address spoofing attacks. In addition, you can configure blackhole MAC address entries to filter out packets with certain source or destination MAC addresses. To add or modify a static, dynamic, or blackhole MAC address table entry.